When i read your post i started thinking about what ideas i could come up with. At first, the fear of not being able to come up with anything clouded my thoughts for a while then i started pondering a few ideas. But the main question that kept coming back to mind was although i may have ideas, i'm uncertain of what's actually feasable/possible within a small project.
Admittedly i've not really looked too far into small team dev projects but i notice most are fairly simple designs, moreso resembling tech demos than anything. That sounds very judgemental, but i don't intend for it to be like that at all, but as i said, i keep coming back to the question of whther or not X is possible.
So i guess i'll just suck it up and throw an idea out there which may be snickered at for either a) it's creativity or b) it's feasability, but hey, the only dumb question is the one you don't ask
I think it's fairly safe to assume that it's a far simpler project to create a mod based off an already existing engine than it is to create from scratch, so with that in mind, just trying to gauge the size of a project with something like:
* Modern-ish FPS engine (ala, Q3/UT2/CryEngine) mod;
* New HUD/UI & Menu;
* New single player map;
* Maybe implement a new weapon/s if the models/textures are readily available;
Keeping things like the original sound files and textures, etc. Whether or not AI implementation is a big job i assume depends on the engine and tools available? I guess the map design is where the majority of creation would come from on a smaller project as developing larger things like a mass of weapons, tricky scripts and whatnot are for a larger project.
Or am i going about this totally the wrong way and should be starting with scratch ideas than to start off with mods or even total conversions of current engines?
Feel free to poke at any points, as i said, i'm just spitballing assumptions here.