Frenkie is an interesting one. He's made out to be the best thing since sliced bread now, but I think young Phil easily has as much potential. When Frenkie was Foden's age (he's three years older), he only had one top tier game under his belt for Willem II. He then moved to AJAX and played for their reserves for a season and a half. It wasn't until half of last season that he stepped up and became a regular in their first team. his development since has obviously been explosive, but still.
As to replacing Fernandinho, I'm not so sure about that. He would fit perfect as a deep lying playmaker, dropping in between the centre backs in our build-up, like Fern sometimes does (he's fantastic at bringing the ball out from the back), but I don't see him as someone that effectively shields the back 4 or put in those crucial tackles to break up counters. He's still a feather weight and I'm also not sure that he has that defensive awareness.