but then you'd have to change every single-player mission, as they're designed for certain speedsThat's only an issue if you replace the original tech files. You can just have a tech tree with a new name, and then only levels designed to use the new tech tree will be affected.
Techtrees can't add units to the game.You kind of can add units.. make the RPG realy powerfull and slow shooting and cost 2000 ore. That would be a new unit. But yea we are stuck with what we have unit gfx/shooting style wise.
I don't think anything can add units to the game, actually. Not without some major hacking of OP2.
And also make maps similar to Pie where you have the basic tech done for you to save time.Pretty much all (Dynamix) maps are already like that, it just depends on your starting resources you choose during game setup.
I don't think we can make things unavailable without some fancy DLL code. There's a REQUIRES field, but not UNAVAILABLE field.Sure you can. Just take the tech that is associated with a specific item (see building / vehicles / weapons.txt) and set:
COST -1
EDEN_COST -1
PLYMOUTH_COST -1