Off-hand, I believe there are two of them in the Serengeti:

1) Singita Grumeti Reserves (with Singita Sasakwa being the flagship Singita property there.)
Not only is the area exclusive to Singita, but the camps there are allowed to offer walking and horseback riding in the reserve, as well as night drives. And vehicles can go off-road, which is not permitted in the (main) Serengeti. Drawback is that outside of the main migration seasons (typically May-June and October-December), game viewing can be weaker than other parts of the Serengeti.

2) Maswa Conservancy
It's in the extreme Southwest of the Serengeti, where Legendary Mwiba Lodge and Legendary Mbono Camp are situated. We haven't been to the Maswa, mainly because the main migration season is usually between December to March, and it's also rainy season... which is why most people prefer to go up to the Mara area between July to October to witness the migration during the Serengeti's dry season. I've had 2 acquaintances stay at Mwiba, and said it was exceptional. Super quaint and private, with great food, service and guiding. And Wildebeest as far as the eye can see (they went in February and March, respectively.) Also a ton of calves at that time! As is the case with Singita Grumeti, vehicle off-roading, walking safaris and night vehicle safaris are permitting, significantly enhancing the experience vs. other camps in the Serengeti.

As an aside, there just isn't a great substitute to the Mara River migration crossings in Serengeti North. It's dry season, animals are everywhere, you'll have a high probability of river crossings and there are several terrific camps in Serengeti Mara (led by - in my opinion - Nomad's Lamai camp, which is phenomenal.) Yes, there is some vehicle clustering when the herds are crossing the river, but once you see the Wildebeest in action you'll be able to block out the distraction of having other vehicles and people around.

For migration action, the other option would be to visit the Maasai Mara during the Northern Summer. Quality is equally high, but vehicle density is much, much higher than in the Northern Serengeti. One possibility is to stay at one of the conservancies and then raid the Mara River area (in the main reserve) to check out the crossings - for every 3 nights in a conservancy, you are allowed one visit into the main Maasai Mara reserve (although you can also pay a-la-carte if you want to take daily visits to the main reserve.) Mara Plains Camp is supposedly tops in all of the greater Mara region - it's in the best conservancy (Olare Motorogi), and is closest to the area inside the main reserve where the Wildebeest often cross. The best of both worlds.