Since my playthrough of Doom 3 and Doom 3 Resurrection of Evil, I've been wondering why I really liked the environment (flashlight gripes aside). It finally hit me in the middle of the main game that a lot of the structures were reminiscent of the structures in the Taris Lower City with the demons very much like the beasts in KOTOR 2. When on the Mars surface, it felt a lot like being on Korriban (think Valley of the Dark Lords). In Resurrection of Evil, when fighting the 3rd hunter, it reminded me of the fight against Darth Revan on the Star Forge or that big beast in the Malachor depths on the way to Trayus Academy. Even with features like the Artifact to slow down time, they could apply the opposite effect for Force Run and then with the grabber gun, it could be used for force push and pull. On the Sith side, the 3rd hunter's "electricity" animation could be used for force lightning.
As far as KOTOR 2 goes, in my recent playthrough while on the Ravager, Mandalore said some stuff that got me thinking. No one knows exactly where he went but that he went into the Unknown Regions so a KOTOR 3 plot would be similar to the first two games where the Exile visits several planets to figure out where Revan went and the final planet is either some mysterious planet or even a Sith stronghold like Dromund Kaas. The final light side battle would then be Revan and the Exile fighting the Emperor, his army or destroying the capital city or something to come back and say the Republic is safe. The dark side battle would be Revan and/or the Exile telling the Emperor about the Republic being in disarray & the Jedi order destroyed so they go to conquer them.
Long story short, it feels like the Revan story could be completed nicely in KOTOR 3 by building a story around the Exile going to find Revan in the Unknown Regions, learning he was at the Sith Empire and bringing him back. The basic functionality is already there of visiting 5 or 6 planets, discovering the final planet and having a big boss fight. We do not necessarily have to take all the characters from the first games but at a basic level, I would take T3-M4 & HK-47 and leave the rest in the Republic to keep things going to try and rebuild what they can or at least not make things worse.