Alright, here's the Wookieepedia page (doesn't tell you much at all): http://starwars.wikia.com/wiki/Korri..._of_Lost_Souls
And here's the Planet of Lost Souls article (it's not too long at all so you could be able to read the entire thing very quickly): http://wizards.com/default.asp?x=sta...031002korriban
No, the story itself is not set in the Old Republic, but the place of course is. We've only explored Dreshdae, various Sith Academies and the Valley of the Dark Lords in the EU, and all of these are also explored in KOTOR, but another very interesting area, the Valley of Golg and the plains around it, built as a burial ground for the ancient Kings of the Sith species, and located a long way away from the Valley of the Dark Lords near the equator... this area is not seen in anything but the Planet Hoppers article, which is a shame. The mod could explore the plains of Korriban and the Valley of Golg (thousands of years before the pirates stumbled along), along with the tombs that are in it... Planet of Lost Souls only shows the tomb of Dathka Graush (we could even have a confrontation with Graush's spirit and his massive zombie army), but I'm sure the tomb of Adas (the first ruler of Korriban) and maybe the tomb of Golg (no one knows there is a Sith Lord called Golg, but if it's the Valley of Golg, it sure could be named after another ancient warlord). This would also be a good chance to cover the actual Sith species a bit, rather than the Dark Jedi and their halfbreed descendants.
Edit: Oh, and another thing. There's quite a few places on Korriban we can see, but we can never actually go there. It would be pretty cool if we had a mod that, say, added the hills with the six giant statues built on them that continues on from the Valley of the Dark Lords. There could be temples, tombs and palaces there. And how about a secret tunnel leading into the Sith city under the Valley which the archaeologists mention? I admit I don't have a clue how to mod (yet), but considering the models for these locations are there (or at least I presume they are), then it probably wouldn't be too difficult to improve it and make it a new area to explore. Korriban is a very interesting planet ande I don't think KOTOR did it justice whatsoever.