I had the idea to make Polymorph like a collection of several metamorphoses :
You have different "types" of polymorph like plant type, amphibian, bug/air, basic,...
All these skills stack with themselves for increased effect (arranged in levels or tiers). The actual mechanism is to have the stacks be removable or nurturable by elemental effects or damage types.
As consequence you basically have a guy who cannot do much for the first two rounds except for taking hits. Your enemies *should* concentrate on removing your stacks before they fully establish/become too many layers to be removed efficiently (there is the CON-synergy). They do so by burning a guy using plant morphs ("Skin of Thorns") or using crushing damage against bug type morphs ("Blood Leech", ?"Wings"?). Obviously it has the advantage of forced synergy.
Novice skills are mainly "basic" and can hardly be interrupted (but they do not make you "unstoppable"). Possible to add some kind of joker move which increases some/all stacks.
One possibility to give enemies incentive to interrupt you is either to make stacks grow per round (if cooldowns are longer) or stacks increase more the higher your current tier is (if cooldowns are short).
They main and gravy problem is that fights usually do not last that long for this version to unfold. While D:OS features fights decided within 3-5 rounds, you would need an ideal 8. Even if, for the most time your overpowering character will crush some barely surviving weaklings.
