Ravenloft: Strahd's Possession, Dark Sun: Shattered Lands and Spelljammer: Pirates of Realmspace are completely different games only by being on different settings. Has similar graphics and almost the same game mechanics but the setting makes playing then completely different

Same with mechanics. Diablo 2 and 3 has a similar setting but are completely different games not just by the wow arstyle of D3, but also by WoW mechanics on D3 and mechanics(which ruins the game as someone who hates things like cooldowns)

Mechanics and setting is what DEFINE a RPG. Putting DOS2 mechanics into Dark Sun would make it a completely different game but a Dark Sun on DOS2 engine with AD&D mechanics and Dark Sun setting can feel like dark Sun.

That said, the unique big depart from Original BG is the turn based. And i an probably one of the few people here who is neutral between RtWP and TB. Both can be good and both can be bad.