Tbh, I'd prefer quality over quantity in regards to encounter design.
Instead of adding more mooks (making any attmepts at tanking even less relevant and overal combat experience even more chaotic), I'd like to see us fight balanced 5-man parties with dedicated roles and stats comparable to a palyer party. With enemy priests throwing clutch sanctuaries on a ranger that's being focused, paladins reviving people, enemy rogues stunlocking your support casters, while enemy mages drop an arcane dampener. That kind of stuff.
Would be even funny to see OP builds with swift flurry/empowered missiles used by AI against players. Such encounters could kinda encourage strategic/reactive usage of skills, instead of relying on spamming the best AOE available and watching endless mob stance procs decimating everything.
Another idea, also this would have to wait till DLCs at least, is to implement some actual mechanics in the combat encounter. Currently, all fights are just tank'n'spank with no other requirements aside from maximizing the damage per second. Would be interesting to see some dynamic stuff happening during boss fights, e.g. environment hazards forcing to repositiong your party, priority targets that require you to stop killing the boss and do something else, perhaps some 'big'n'bad' abilities for bosses that should be countered in a specific way, etc.
Simply adding hp/damage/mobs is always the worst possible answer (aside form not doing anything at all).
