I'd like to see an actual defense robot that doesn't de-spawn, but can only be used in roboports. It'd be cool have an army of flying machine guns come to the aid of the turrets. Or maybe a a laser upgrade to construction robots, I find that they die a lot when repairing on the front lines.

I've thought a bit about belt upkeep as well, and I think the best way to balance it would be a "belt motor". This would function as a regular belt, it would have all 3 tiers, but it would have a slightly fancier texture. This belt would need power, or perhaps fuel, and any belts connected...

they could also be used in farms when food is eventually added. Although I would think that they would be a perk for having low pollution

This would be helpful, especially when figuring out what biter spawners are being polluted and attacking you

This actually looks like fun. I'm going to try something like this