2 vs 2 ... and other Team Games
Some rules to suggest
Consider when doing team games, that victory points are combined. I.e. both players in combo need 20 VPs, not each player get to ten (this way you are helping each other, not playing independent).
Consider having roads be shared. I.e. if i build a road, you could build a road, then I could build a settlement. That way there is true coop play. However, settlements and cities are your own. So you couldnt put a settlement on mine.
Teams shouldn't be allowed to trade with each other. or you'd never trade with opponents.
If you like any of these, I can go on and on.