Key part :

I wanted to make sure fans know this is a really embryonic process. I haven’t even seen outlines. In the press at large, everybody said, ‘there are four spinoffs’ and they assume that means each one is happening and we’re going to have a new Game of Thrones show per quarter. That’s not what’s going on. The idea is not to do four shows. The bar set by [Benioff and Weiss] is so high that my hope is to get one show that lives up to it. Also, this is a long-term plan. Our No. 1 goal is the seventh season this summer and getting the eighth season written and aired.”

and then

If you only developed one, everything would rest on that one shot. It’s such a special show. I want to make sure that [any prequel] feels worthy. We have some amazing writers who want to take a shot at this. They’re also looking at different times in the universe and all will have different feels. This increases our odds of finding one that’s unique.

also

You’re not going to see a situation where the next show in the Thrones universe launches off the back of this one. The show that Dan and David have created will get its proper send off first.

and then I found this bit interesting :

*Q: I heard originally that one or more might be a limited series instead of a regular series. Is that anthology-style format possible? *

A: At this point, everything is on the table. The idea is to find a series. It would be nice to find something that has the legs this one did. But if something works better as a limited series, sure.