What I'm really worried about is that if my Solasmancer Lavellan drinks from the well any future content with Solas will be all f*cked up because of it. I don't want her "enslavement" to Mythal preventing her and Solas having at least a sort of happy resolution romantically.
Thoughts?
I think it might just be a case of "Static Role, Exchangeable Character."
That is, at certain points in the story there will be a character to fill that role no matter what, but depending on the player's past choices (in previous games) it could be one character or another.
For example, in DAI Varric brings Hawke no matter what, and Hawke has a "Grey Warden Contact" to give the Inquisitor information on the Grey Warden's activities no matter what (Static Role). However, who that Grey Warden Contact is depends on which Grey Warden companion fought beside the Hero of Ferelden during the Blight and/or whether that companion survived (Exchangeable Character). DAI Hawke's "Grey Warden Contact" could be Alistair, Loghain, or Stroud (if one of the other died). The story then plays out normally.
Likewise, Morrigan shows up in DAI to give insight into ancient elven lore (even for a Dalish... grrr!) no matter what, and I think eventually encounters Flemeth no matter what (Static Role), but whether or not she has a son and what her son is like depends on the Warden's decision in DAO (Exchangeable Cahracter). If the Warden chose the Old God Ritual, Kieran has the soul of an Old God. If the Warden was male and romanced Morrigan but didn't accept the ritual, then Kieran is a normal little boy. Depending on who the Warden chose to do the ritual (and gender) Kieran could be the Warden's son, Alistair's son, or Loghain's son. There's different dialogue acknowledging him and his personality, of course; Morrigan can be much nicer with him around, and her encounter with Flemeth alters depending on whether she has Kieran and/or the Old God Soul, but ultimately the story plays out similarly.
Likewise, whoever drank from the Well uses its power to summon a dragon to fight Corypheus' dragon (Static Role), but who it is and how they do it depends on who did the drinking (Exchangeable Character). If Morrigan drank from the Well, she's forced to transform into a dragon and fight Corypheus' pet head-on. If the Inquisitor drank, then s/he goes to visit one of Mythal's temples, summons a dragon, defeats it in combat, then uses the Well's power to enthrall it to her.
Likewise, if the Drinker of the Well comes back in a future game, the role will be static and the character exchangeable. It won't be too different depending on whether it's the Inquisitor or Morrigan; just different dialogue and maybe some conversation paths.
My guess is that either Mythal's "spark of godhood" that she hoped to pass onto Morrigan and which she safeguarded from Solas will thrive, she'll come back and do something perilous, then will call upon the drinker of the Well as her servant in the fight. Or, Solas really did absorb part of Mythal's godhood when he absorbed her power at the end and will also be able to control the Drinker of the Well. My guess is that he'll try to do something earth-shattering, the new protagonist will have to try to stop him (whether by trying to talk him down, kill him, or defeat him in battle and then try to talk him down), then he might feel forced to summon the drinker of the Well for help (though I think that'll be an indication that he's truly desperate since he doesn't believe in infringing on free will), and try to get the Drinker to fight the new protagonist; whether the thrall is the Inquisitor or Morrigan just depends on who did it. The player will defeat them (hopefully not kill them), then probably stop Solas' plan, but hopefully not have to kill him. (Creators willing, we'll be able to talk him down the way the Inquisitor could talk Abelas down.)
I hope we aren't forced to kill the Well-drinker out of hand (since I don't want to kill Morrigan or the Inquisitor), nor be forced to kill Solas if he gains some control over the Well-drinker (oh Creators, please let us be able to talk him down like we could Abelas!), but no matter what, if the Well-drinking Inquisitor is a female Lavellan who romanced Solas and Solas somehow holds the leash, I hope the devs remember and keep the exchange very well-written and emotionally compelling; just like how they kept Morrigan's reunion with Flemeth in DAI well-written and emotionally compelling whether Morrigan had a son, a son with the soul of an Old God, or no son at all.
Even if it ends tragically though, I probably won't change my worldstate. Probably. Not unless it's needlessly, sadistically heart-wrenching.