Well, you'd have all these 'powerful' jutsus at your disposal, and any other benefits in game from those KGs.
Also some wouldn't work together, and it makes sense that you can only have one from each category.
Just to keep you settled I'll reveal the details of Sage Arts, which are final yet.
The two new KGs will be Sage Arts, Rinnegan and Sage Mode.
You can only have one Sage Art, and if you have a Sage Art, you can't have any other KG.
As such, they may be slightly more powerful than the average KG, and contain 6 resets each.