Ok found some more info... they're planning a track between 2.8km and 3.5km and a "Level 1" kart track.
If by level 1 they mean grade A it would have to be at least 1.2km long and at least a 150m straight. It would also have to have a full servicing park and a lot of area for competitors' tents among other things. If it really is a grade-A track then there will be a really nice karting track there as well.
So the big track is for non-F1 stuff, like smaller single seaters, saloon cars, bike racing etc.. crowd capacity at 15,000, so I doubt it would be hosting anything like MotoGP with only 15,000 seats. A small track is what you need though, it'll create good atmosphere at small events because even with 5,000 it will feel like 50,000 at Sepang.