Depending on your perspective, there may be zero, one or two next levels after Platinum Elite.
If you're looking for a level where you will earn a higher bonus percentage, more tangible benefits, or significant recognition...then the answer is most likely zero. Marriott's highest official Elite level is Platinum.
If you're looking for a Platinum "plus" category.... then the answer may be one. There is a designation for the top 3 percent in the program known as Platinum Premier. There are many threads on here that highlight the somewhat secret category that doesn't have a lot of perceivable differences from standard Platinum Elite. No additional bonus earnings but there is a dedicated phone line and the theory that if a Platinum and a Platinum Premier both checked in at the same time and one upgrade room was available, the PP would be given the nod.
If you're bored and wondering what else you can strive for..then the answer may be two. Besides the Platinum Premier category there are also Lifetime Elite levels. Silver, Gold, and Platinum. With your 700 nights you are closing in on the qualifying number to achieve Lifetime Platinum. You'll need 750 nights along with 2 million points earned. Lifetime Gold and Lifetime Silver need 500 and 250 nights along with 1.6 million and 1.2 million points, respectively. Once you attain the thresholds for Lifetime Elite, you no longer need to qualify annually to receive the benefits associated with each level. These lifetime designations are a real nice feature of the Marriott Rewards program