I don’t like POE flask system. Piano is stupid and overcomplexity (+1 to arrows, add resistances, remove burning, add run/attack speed, add ele damage, add critical strike, etc etc) is just bad. I don’t want all these on flasks. And why on earth do flasks fill up when I kill mobs, where is the logic behind that? Don’t like it too, even from the gameplay perspective (how it plays) (those mob waves on bosses just to fill flasks? omg, brilliant…)
D2 system is strange and funny but it works in its own strange and funny way.
D3 system is the best - simple and elegant decision. But I would make potions tiered: white, magic, rare, unique.
Small white (ordinary) potion gradually restores, lets say, 50% life. Medium - 60%, Big - 70% (numbers should be close for further possible choices to make).
Magic adds special potions affixes (similar to POE magic flask). Like +5, +7, +10% more life to restore; or +40% to restoration speed; or 10% of mana pool is also restored; removes poison, everything like that, but nothing like “+1 to arrows, to damage, to shield block”.
Rare - more affixes; Unique - unique affixes, maybe smth valuable like “3 sec immunity to fire” or “2 second invisibility”.
The very idea is that this is always a healing potion with a permanent cooldown that always restores at least 50% hp, but can have a bunch of useful secondary affixes as a bonus (always connected with defence and survivability). And all bonuses are activated at once, when you drink the potion.