Personally my biggest question marks right now are the OL situation and the new special teams coach. The OL is priority one and I really think, if improved, it solves not all but many of the questions surrounding Gurley, the running game and Goff. They had a top-3 OL for two years and then basically a bottom-5 one last year (with some glimmers of hope). They need to field something that's at least middle of the pack in 2020 and ideally be somewhat healthier (25 starts missed in 2019).
Defensively, yeah, holes will be created in the front seven. Only one- or possibly none- of Littleton, Fowler and Brockers will return.
But they do have AD and a really talented and deep secondary. So they're not starting with nothing or turning over 7-8 starters.
I do think 1-2 quality new additions on the OL and front seven will yield strong returns. Easier said than done of course, though.