I've played this mod for a bit, and i have a lot to say.
It has decent gameplay, amazing music, but it lacks a lot in replayablity.
Personally, 6 weapons is simply not enough to make people constantly come back(Yay let's do a dagger playthrough! I love pain!). Addition of classes might change that. Each class having 2 weapons. Primary and secondary. Each with a few choices for each weapon. Classes with ranged, melee, throwable, mage, summon weapons should be added. Maybe also make some simple armor for each class. Not sure how, not sure if its even needed, but it would be a cool addition too.
Yes, you read correctly, items. While the passive items are a very easy way to make every playthrough a bit different, there aren't enough items to make every time feel different enough. Adding more of them would be nice, but what would really elevate the difference in every attempt is... Usable items! They would give the player little buffs, healing, deal damage on their own when used... Only imagination is limit. But yeah, it kinda sucks that there are only passive items present...
Room structure is way too linear. Maybe add some extra rooms that have some other meaning to them, maybe with stronger enemies but you get an extra item. Or alternative paths where you fight other bosses that are slightly harder but also offer a choice between 2 items instead of one, yet you still come back to moonlord as the last boss. *ahem, The Binding of Isaac**
Honestly, i just want to see more variety overall. Room pathing, bosses, weapons, items... Basically everything.. My suggestions take insperation from the binding of isaac, but that's only because it is pretty much the best roguelike games i've ever played. While i did find this game-mode kinda mod quite enjoyable, it certainly lacks a lot of replayability, and there is a ton of things that can be added to this mod, or as separate mods that together make everything perfectly balanced and beautiful.
It's great, but there is always room for greater