Yeah, if I were to guess I would say A17 Pro (maybe A18). The A17 Pro was the first SoC with AV1 hardware video decoding. Seems unlikely (to me) they would neglect a rapidly growing video codec in their video decoder box. The A17 Pro also has significantly better game support when it comes to fidelity, much faster GPU, ray tracing support, etc....
Since Apple will focus on features (not specs) they will use the cheapest hardware available that supports whatever features they are cooking up. A18 has a physically smaller die size so the yields are higher, I don't know if that's a meaningful cost difference vs other variables though.
The SoC in the ApplleTV is already overqualified, not only is my 10 year old AppleTV HD up to date but I never see my AppleTV 4k (gen 1 A10X) exceed 40% CPU utilization. Any stutters I see are frame stalls from an unoptimized apps or questionable design choices in those apps (network calls while scrolling, proprietary players doing what it is they are doing). Even though the SoC is a lower tier bin there are half the processes and a fraction of the services running on the AppleTV vs its iPhone counterpart.
I'm more curious what they can possibly do with a new AppleTV that isn't anything more than an bog standard incremental update.