Interesting yet odd idea for a game with asymmetrical teams, and it would require a whole ton of refinement. Right now, it just sounds like a rough workshop mod.
I imagine new building models would have to be created to make up this “base” that would also have to fit in appearance-wise with the rest of the environment. There would need to be new voice lines, custom maps, animations, UI changes, et cetera, on top of extreme balancing so it doesn’t become another Nest / Rescue. This is just for a rough idea on the amount of work and money this mode would take.
Now, on to the gameplay. You need incentives for both sides to play out their intended role (much like the timer and relay does in Hunt), but also make it fun. Would it be fun as a Hunter to just camp at the base once it’s upgraded? Would it be fun for a Stage 3 Monster to take on 4 hunters with extra shields, extra damage, medic bays, and/or extra utilities? Would it be fun for four hunters to take on a Stage 3 Monster with 2+ minions, even with base defenses?
Also, what is the end goal for both sides? If it’s point-based, then the hunters will just camp their base to prevent the monster from getting good points, as he’ll always have to go to them to gain the most points. If he avoids them and farms points on wildlife, the hunters will have to go stop him - at which point he can just lure them near an egg, hatch a minion, and wreck their face while they’re away from their base.
What happens when the monster dies, if the hunters get unlimited respawns and super base defenses? Does he have infinite health?
There’s a lot of factors that need to be considered because of how Evolve is so vastly different from other shooters or MOBAs. Looking at a game mode from DOTA, Smite, Battleborn, COD, or any other PvP games and “Evolveifying” can be pretty complicated and difficult, if you really think about it, and IMO it won’t work too well. I think, to come up with a new game mode for Evolve, it really has to be thought of with the original concept of the game (Hunt) in mind, thusly balanced around the idea that it has to be 4 v 1 - or at least involve both monsters and hunters.