I chose engineer and Construction Yards.

I just want to stick what the original game does, so capturing tech from other buildings with thief or spy feel strange to me.

As a mental omega player,I prefer thieves&spies.

I don't really care either way. I'd say just send the spy into the construction yard

A spy into a tech center would be the most logical choice I think, though in RA2 that unlocks one special unit rather than giving you all enemy technology.

The in-game description when you hover your mouse over the Spy icon says it can sabotage enemy buildings, but actually it can't do anything like that. So maybe that would be a cool feature to add?

So while infiltrating a barracks or war factory could still let you see what they are building (which is pretty useless), perhaps it could also halve that type of production for 30 seconds, as if the base had low power.

And perhaps infiltrating a power plant... shows you how much power they have, but also stops that particular power plant from producing power for 1 minute. (Less powerful than the RA2/OpenRA version where it completely shuts down all base power.)

Radar dome... disrupts radar (like a radar jammer/tesla tank) and resets shroud. Obviously that's only useful against a human player. (It's also already supposed to allow you to see a special radar screen that lets you see enemy movements and much stuff they have, but that's never worked for me.)

Oh, just had an idea... if you send a spy (or maybe a thief?) into a war factory, airfield or naval yard, the next vehicle produced by that factory will be on your side instead of theirs! This would be similar to how in the second Allied counterstrike sarin gas mission, you can send a spy into a Soviet war factory and that lets you then drive a Soviet tank.

So, if an enemy is building a tesla tank when you infiltrate their war factory, in a few moments YOU get a tesla tank in the enemy base instead! The spy or thief has hijacked the vehicle. This lets you cause some damage, AND it wastes their money!

That way when a spy infiltrates a war factory the little picture telling you what they are building will be useful, it'll be a preview of what unit you are about to take over.

It would also be great if thieves could have the same disguise ability as spies. Thieves aren't available until the player has a tech centre, despite the fact that spies only need a radar dome. In RA2 and OpenRA they got rid of thieves and instead gave their ability to spies... if thieves and spies are going to be seperate units, perhaps thieves should be like "spies that can steal money".

@your evil twin:
Interesting ideas, but not easy to implement.
I will have a look at it but can't guarantee anything.