Well, what would make things harder would be to decrease the general amount of ore on the map. This can be done either by fewer mining locations or lower yields.
Personally I like the evacuations. Getting the right spot for your base should be made more important I think though. Now you can easily settle for a 3 bar and rare is your only concern. But then we end up back the the first point, less ore.
I think a more varied cooperation level could be good. some maps where you are dependant on co-operation, others were you would have to fight some. (Less mining locations would force people to fight for their survival perhaps?)
I like the storyline of OP2 and playing through the campaigns I used to like the reinforcements coming out of nowhere aswell as the post-release demo where you had to collect a spaceship part. So personally I would like to see a more thorough build up maybe, with players recieving additional buildings vecs as reinforcement. Spaceship parts could also be scattered around the maps for free spaceship elements or free research ? Dunno what is possible here though ? ** Got some ideas here which can be discussed, but I am not sure what actually can be implemented **
Generally, I would focus on leaving players with less ore to toy around with. That way everything would be more valuable and survival harder. Just my 2 cents.
EDIT: I also like the enemy to have a base we can attack. Spawning points is just annoying. Something concrete to fight and be wary of is much preferable to loads of units just rolling into the maps.