Ooh that’s an interesting thought. I never played around with it cause I thought the two modes were enough to manage, but there are a few ways I could try to make it feel natural. I could make it so a simple left click does a normal attack, and holding it while diving lets you charge up a dive attack that scales heavily with speed but restricts your maneuverability. I’ll have to play with the idea more after work tomorrow and try some other stuff, but it seems like a nice balance if I do it right, and I’m glad you brought up how it affects regular flight! That is the most important part of the mod after all, that has to feel great and the rest of the mod is built around it

played around a bit via spawning in dinos with admin commands.

critiques:
griffin: you can't stand on it's back. it has a slower max speed and a wider turn radius when compared to KBD's counterpart. you also can't land at full speed and keep running. you also are looking through it's model in first person view.
quetzal: stamina drain is absurdly fast, given how difficult it is to gain altitude and that it has a max speed that is only 2/3rds of the base speed of most other fliers. I haven't been able to figure out how to get fast enough to soar with a quetz.
snow owl: doesn't turn into a IF flier when fed the evolver, so I haven't been able to test. it instead adds another evolver to the stack, same as feeding the evolver to non-IF-able dinos. don't know if you can duplicate other items with this, but if it only duplicates the evolver, that's not that bad. it's just one stimberry.
tapejara: can no longer strafe in either axis. also, flies far too fast for a tapejara. and it gains altitude incredibly well. like, it felt like it climbed the best of all of the ones I tested. this is a fighter jet, not a tapejara. you also are inside of it's model while riding. like your position is what I assume is the exact center of the body. kiss first person good bye, and be careful where you land, cause you may need to crawl out.
wyvern: slowly increases speed while flying till it reaches a cap. the starting speed is around a fifth or so the speed of the cap. this makes it feel like a supersonic jet. it feels great. you are able to turn about as well as a wyvern normally does at that speed too. I'd reduce the turning ability a bit dependant on speed, but other than that the wyvern feels great.
pelagornis: a bit hard to take off from the water, but very doable. the land flier whistle didn't do anything. I didn't try with any of the others, but I'd assume it'd be the same there too. problem with the pelagornis and by extension the pteranodon. once they land in water, they bug out while landing on land. the pelagornis swims as if the ground is water. and the pteranodon uses their soar animation and doesn't get stamina back until removed from the world and added back in (cryo, soul ball, your choice).
pteranodon: a great baseline for all the other fliers. it moves at a speed where you have to react quickly to things like terrain and trees, but you still can react (unlike a few of the others). turning it in the air is very satisfying, you can usually move it exactly where you want. speed control is reasonable. you never go too fast unless you pump the speed stat (minor increase in max speed and acceleration, 150% adds like 5m/s over 125%)
lymantria: a good trainer flier. it flies almost the same as a vanilla flier, with a few new elements from IF like variable speed. seems great for getting people used to IF if they don't want to jump into the deep end. very loud wind effects when flying though.
argentavis: this complaint extends to all the fliers above as well (other then the quetzal, the pteranodon, and the lymantria), it accelerates too fast. you go from idle to max speed in a second or two. there is little reason to fly fancy, and since I didn't level speed on this one either before or partway through, it's max speed is too fast too. since you can reach it in two seconds, you move across the map too fast. the argent is twice as fast as the ptera on a sprint. that should not be the case. endurance race, sure. but not a sprint. as stated above, this goes for most of the other fliers as well. (all the fliers with this issue are also really really loud)

recommendations
use the ptera's current movement as a baseline for the dinos rather than the argent, ptera feels better to fly and is more balanced. let the tapejara strafe and reduce it's max speed down. make the griffin turn better and able to land running depending on the angle of impact. increase the max speed and climb speed of the quetz. fix the bug where if a ptera of pela has landed on water it now bugs out when it lands on land. add functionality for the "land this flier" whistle.

personal wants
Phoenix plz, plz, plz, plz!!

Seconding owl not working (tried both variants), and standing on griffins backs.

I was about to say I haven't had any issue with Ptera and Pela not landing properly after swimming, but I just managed to replicate it. It happens if you swim onto land rather than take off from the water and then land, if you take off from the water they land fine afterwards.

On the topic of things BD used that might be of interest for potential use, full pitching for Quetzalcoatlus might make it feel a bit more in line with the other flyers, and the wyvern has an unused roar/emote animation in the dev kit.

I did forget to add Owl to the Evolver list, so it should be spawnable but not evolvable yet, will be fixed as soon as my DevKit loads >-<

For some of those Lia, I would just like to point out that if you brake, below a certain speed both Lymantria and Tapejara can hover, plus did you test these flyers with weight on them or completely empty? Your current weight will affect how they fly significantly. Also Quetzal's speed increases with altitude, it is encouraged to take off from big rocks or cliffs and immediately climb as at 200 meters it can easily maintain 27 m/s and still gain stamina (I'm reducing this to 150 meters as I make this edit). I very much appreciate your input, but I just know you said you were new to the mod and wanted to clarify a couple things that I haven't documented well yet.

I'll definitely be tweaking some things though, Tapejara I wanted to be unique in that it has incredible acceleration but a notably lower top speed, clearly I set its speed a bit too high. I did do some major rethinking of each flyer's role in the mod while I was on hiatus, but that means I need to refine the stats again ex. I wanted Ptero to be a more basic flyer like Lymantria, but I think Argy is a bit too fast itself. I'm glad you liked the Wyvern though, it definitely turned too slowly before

...plus I need to figure out how to stop the land swimming lol

thanks for the clarifications! I was doing this with only the base character weight (35 I think) and whatever a saddle weighs if it even does. it's really cool that carry load plays into the flight mechanics!! I look forward to your documenting how things work or someone writing a guide, there's likely a lot of other stuff I missed. also, by "break" I assume you mean the dino airbrake ("x")? I personaly only really use the dino airbrake on vanilla fliers for fine control while building. but that's more of a "me" issue since I haven't bothered to rebind it to somewhere sensible (it's such a pain to reach). the ptero was really satisfying. but yeah, argy was tangibly faster than the ptero. it made me sad cause I liked the flight mechanics of the ptero better.
my computer went kaboom and I've spent the day trying to fix it, so I haven't played any since writing that, but I definetly will when I can.

I'm glad to see you working on this and talking with the users, it feels really refreshing :)

Oh, if you hold space while you flap that's your airbrake now. It lets you land more precisely, slow down to hover speed, and avoid collisions in some cases, cause you also turn faster when flying slower. I rebound it in the mod since I'm using X for the Auto Align toggle and that is a terrible spot for an airbrake haha

Hope you get your computer fixed soon!

The owl feels absolutely amazing to use! It doesn't have its divebomb, but you can replicate its effects by holding secondary attack while flying, felt odd it had two ways of doing essentially the same move. And standing on griffins! So happy to have it.

A friend has reminded me that I forgot to mention earlier that griffins don't have a damage overlay, and neither they nor wyverns have IK (leaning when they're on slopes). Though I'm not sure if the latter was actually a vanilla feature or not, if not please excuse.

Seconding good wishes on fixing that computer!

*edit*
Owls also don't have damage overlays.

Encountered another bug. Ice wyvern textures get weird when you convert them. The colours look too "strong" and the texture looks washed out and less detailed up close.

Converted wyvern above:
https://i.postimg.cc/13qDwpJs/dsfsdgasdvavs.png

Would be nice if a few more of the flyers would gain the ability to dive in water, even if they were worse than the pelagornis (which they should be), maybe as a configuration option? Would be an interesting option for crystal wyverns for example, as they're the small tropical cousin it feels like they should

I think it would improve QoL if you only had to press spacebar for the air break
Edit: Would be cool if the flyer would only dive when you press S

A suggestion I have for this mod is to add a landed sprint to the argy, and to other flyers who lack it. I've noticed this mod adds a sprint to a few creatures like the quetz, and I think other flyers, the argy in particular, could really benefit from it, as it moves painfully slowly while landed.

I love this mod, the only thing I greatly dislike is that it allows IF flyers to enter caves, no matter the server settings, making caves rather trivial. I've been bashing my head against this problem for a while, but as far as I can tell, there's no way to edit a list of creatures who are/aren't allowed to enter caves.

@Stilgar; Is this possible?

I hate to tell you this, but I'm fairly certain Stilgar has moved onto ASA, as have many ARK modders. Personally I'd suggest you just don't take IF flyers into caves if you want a challenge. No one is forcing you to do it, yes?