When i play usuall map humans vs bugs i see the poor humans base with generators and walls, sometimes 1-2 bunker. For defence bases humans can put the mine and this is all thay can to do. May be enter some Innovation in humans bases? Some helpful building for defence like sentry guns, force field generator, armor charge computer and so on. But not to give in the beginning, humans must build it. I see diffirent way how they can do that.
First way - enter classes (skills) like engineer, miner, hacker and so on. Its trite, but working in a lot of games. And for each type of building need some human with special skill.
Second way build this for a price - gold, special points taking when you win game or take achievement. May be buy in shop or take from boxes some part, needed to build something. Of course each part only for one or two type of building.
Third way - quest. Like dropped from a helicopter (next to the hive) to pick up parts and bring them to the base.
Fourth way as laying bombs. But available in 5 min of game or later.
Fifth way give special buildings as compensation when leave 1 human, lose 1 genrator and so on.

I think this help make the game more interesting.
P.S
Sorry my bad english, i'm from Russia ^_^

Yep, i understand thats is global idea, but its really fun and new expirience. All we can see turret in Hard Four map, why not add it into usuall map?

good idea and battleswarm have already turrents in hard fought, maybe something like turrent construct where team acumulate construct poins killing bugs and side bugs conctruct burrows