You guys are making it seem way harder than it is. Multiple RPGs released the last couple of years use additional monsters per encounter to tune - best example being Pillars 1+2.
To be fair, Pillars still also relied heavily on stat buffs on higher difficulties. Yes they added monsters (compared to base difficulty), but PoTD in Pillars 2 gives enemies +25% health, +15 accuracy, +15 all defenses (+15 deflection, +15 fortitude, etc.), +2 armor, and +2 penetration for example. Armor/PEN are particularly huge in that system. Veteran also gives buff but smaller. In general, I think as a minimum you want to do a bit of both. Simply adding monsters will cause players to flavor AoE spells/effects, whereas just relying on inflating stats often requires you to increase the numbers so much that you start restricting many character/ability options.
One potential easy solution to spice things up for high difficulty in BG3 is simply just to even the playing field between player and monster. I.e. remove the Death Saves mechanic from 5E (aka Yoyo healing).
Right now, every fight you go into, it's highly asymmetrical and the player has a HUGE advantage. The player party is actually extremely hard to kill, since they don't just die when going to 0, and can easily get back up. They're almost like 4 heads of a hydra that you have to sever all at once.
On top of health buffs, DOS2 did give certain enemies additional immunities/abilities, which I do hope they bring back for higher difficulties in BG3.
For example, in tactician mode, certain Act 2 enemies gain various auras that did change up the dynamics of the fight. I.e. Executor Ninyan gets a perma evasive aura (90% dodge). The Voidwoken underneath the Driftwood fishery gains fire immunity, etc.
So this does make me hopeful that we'll see more than just HP increases.
