4,100 BBY was more than 100 years before KotOR, which first introduced Manaan and Kolto, so even if that's when the galaxy started using Bacta, it's doesn't look like that's when it replaced Kolto.

The timeframe for Kolto finally getting phased out is still up in the air, and they could even use this game as an opportunity to expand the story of how that change happened beyond just "Bacta was better".

 

Personally, I'd really like to see Onderon/Dxun.

This. We know Sith have a history on Onderon, not to mention the conflict between the Beast Riders & Iziz.

Manaan would be cool, but more than anything I'd like to see Dantooine. It's just such an iconic Old Republic-era planet in my opinion. There was also something about "mysterious cloning experiments" at the Jedi Enclave, which we never really saw explained at all as far as I'm aware.

It would also be a great opportunity to explore Haruun Kal, given that the Korunnai were believed to be descendants of Jedi who crashed on the planet during the Exar Kun war (granted, Dromund Kaas and Dxun together would probably be enough of the dark-side-corrupted jungle theme), so I'd lean more toward Manaan in its place).