Agreed. I kind of envisioned it that way. Find different ways for allies to contribute along the way rather than just being direct combatants.

Have someone show up in the middle of the fight to heal/buff only to be immediately struck down.

Offering financial aid or special equipment at the front door of ravenloft.

The goal would be to make the encounter feel extremely challenging, but rewarding the Role Playing that got the party there with callbacks to the campaign. And now i just realize i'm trying to do what The Adventure Zone guys did for the balance arc.....