Spelljammer could be an expansion pack for BG3, pursuing Mind Flayers in another planet, or maybe a whole game, traveling in Star Trek mode...exploring new civilizations.
With comparisons to Kotor..possibly.
As for Planescape, the obvious benchmark would be Planescape: Torment...a masterpiece of older RPG games in my humble opinion.
Then...there is Ravenloft..good environment for higher level characters..loaded with powerful equipment but perhaps unable to sleep well
and recover. The first challenge there could be to find a decent place to sleep.
Maybe the Vampire lord is a high level Warlock which rides his flying black horse and throws energy blasts from above..talk about Larian´s verticality combat..
but then, I don´t think WoC would agree on that plot ... not yet.
