Playing monster has a distinct lack of appeal to some folks and I think the key reason is that most people find it funner to play with others rather than alone and against others. There’s a certain appeal to being on the team that takes down the big beasty, less people are interested in being the loner. I think the fix would be an alternative game mode style that allows for two player/AI monsters. The social aspect is just too strong. This is L4D with a 5th wheel and no one likes being it.
I’d recommend an alternative style skirmish and evac in which there are two player monsters. Call it Pack Mode or something. They do not “evolve” but instead grow stronger the more they feed. They simply “grow” over time and rather than getting additional skill points their moves get better, their damage improves, and their health cap increases with appropriate health regenerated. You’d have your initial three skill points so you’ll be giving up at least one skill. At the start, the monsters are effectively a Stage 0.5 and by end game effective Stage 1.5 with Level 2 skills. Incapping and killing hunters do not “grow” the way they give Evolve experience, you must eat the corpses, but a hunter corpse would give the most growth of any feed. At cap, they gain no more health for eating. Armor generated is split in half to go towards health as part of the growth style leveling and likely would retain the halved armor generation for balance reasons.
Another mode that would be great (and as an available Coop PVE only) that I know has been suggested would be an endless defense against wave after wave of monsters. Allow players to enter in as minions. Similar to L4D where the Tank comes in during a VS mode, a player monster is randomly chosen to play a “proper” and staged up monster rebalanced for the game mode (which would require a variety of additional gameplay elements for hunters anyways).