Given how much they've shrunk the Mac mini down, the next Apple TV will either be a surprisingly powerful gaming console in its current form factor, or they'll keep its performance roughly equivalent to how it is now but have it in a far smaller form.
My money would be on the latter. Smaller devices mean more can be shipped for the same cost in less expensive packaging, and that's the Cook Way. They've no real interest in offering 'proper' gaming on ATV, so as long as it can work with 8K signals it'll be powerful enough for what it needs to do.