I am a modder, I developed a dynamic wave generation system last year.

[Lirania]

Hell maybe add different attack locations which are randomly rotated through. Where you maybe get some seconds to move over to the Area, just like in Orcs must die,if anyone knows that game.

Never played that game, but you're talking about different bot spawns, so more than 1 or 2, right? Pretty interesting idea as well, but many of current NI maps might be to small and narrow. 
Switching spawns will make it even more challenging for the team, as everyone really needs to participate, especially if maps won't give good possibilities for a base with shieldwall etc. Just imagine a slightly modified Cav map, that doesn't have a proper shielding spot.

Well to get it a more clear taking Orcs must die as an example.
Its a mix of tower defense and third person shooter.
You have a Portal which you have to protect from Orcs,they spawn from different Locations using different paths. So you have to spread your defensive buildings around on the paths and move around. But that means we would need more cades aswell.

Not sure how this could get balanced. Its just a idea anyways.