I can add a custom area where such challenges away from the main board may fall. can you clarify rulesets for 100% and possibly Spaceport, in case our understanding is different ? we have to be careful about adding too many arbitrary categories, due to overlap and board clarity.

Yeah so if you wanted to add an all reward buildings (there's 41) it would include the space port, I don't know why it would have it's own category. There's essentially four categories of unlockable buildings: reward buildings, business deals, nonreward civics (police, fire, and grand train station - this one might be harder than all the reward buildings), and the small/medium/large airport. To 100% it i figure you'd have to unlock and place every building.