Yeah I wanna setup Thievery eventually for sure. Have a Thieves Guild building (or maybe just expand guard towers to include this role), thief techs that increase the kinds of operations they can run (like spells, but for thiefs).
Keeper has mentioned creating new units is very difficult, but I have some hacky ways around it - like making the building naturally train new thief's each tick - I'd be aiming to make the thief system self-contained outside the rest of the codebase, kind of like the Market, so it will be incredibly easy to expand and have much lower chance of regression bugs.
They can work similar to wizards, you'd use your own thieves to defend against enemy thief ops. Maybe tie it into BAS as a sort of buff/debuff.
I'm excited to get to the brainstorming stage and break ground on it, but not really sure how far out this kind of idea might be. Still fun to think about all the possibilities.