Best case scenario and also the one that is the most unlikely would be Coldzera joining as IGL, he hasn't really been able to prove himself as IGL since the MiBR just is not meshing well but it is pretty clear that he is still hungry for wins/titles and said on multiple occasions that if SK/MIBR would not improve that he would leave even before the americans joined.
FalleN doesn't really seem hungry for wins anymore and neither does Fer, strong possibility that Cold wants to leave but can't due to his contract. ChrisJ would also be a decent option to pick up as an IGL imo.
This alone would not "fix" FaZe I think. I think they need a second dedicated tactical coach. From what I've seen RobbaN is great to help with the mentality of the team and making sure they are all in shape etc. but he is less tactically minded. If they had a good coach that could work together with the IGL to come up with decent tactics etc. then I think they would perform far better. I think they would either need to go for a cerebral ex-player or for a already well-established coach. If you go for an ex-player then maybe it would be better if they dabbled in analysis as well but of course a former IGL would be preferred, essential even. From the top of my head the people that come to mind:
Threat - ex-coach of NiP, has done some work as analyst a while back as well, has proven himself as a good caoch as NiP were performing well with him before the coaching rule was implemented.
YNK - If he got the opportunity to coach Niko I'm sure he would take it since the two have established history together. Haven't really seen great results from MiBR since he joined but that may not be his fault entirely since the team isn't meshing well. Would be interesting to try him out.
Pronax - It's clear that as a player he can no longer compete at the top tier but he might prove to be a good coach if given the chance.
He also had very impactfull midround calls back in his Fnatic days. Especially since his entire carreer he has been IGL, on top of that he has history with Olof which might prove useful.
Sgares - I think it would be a great fit due to his laid back personality but also his good IGLing. It's clear he spends a lot of time analysing games and thinking about the game on a deeper level. Personally think he could be an effective coach.
Starix - A cerebral player that just like Pronax is done in the top tier of CS. I don't know how well he can communicate in english but back in the day the combination of Zeus and Him in NaVi worked very well. On top of that he has history with Guardian.
And finally in my opinion they need a dedicated support player who can keep up with the performance of the other guys. I don't see a lot of options here but KRIMZ would hands down be the best pick for them. Not only is he a dedicated support player, not only does he put up insane numbers and often even carries his team lately but he also of course has his history with Olof where in the old Fnatic they were actually known and feared for their almost unbreakable defense as a duo on a site. I think it would be very interesting to see KRIMZ join up with Olof again. Kioshima is also a good option I think. Now if there is no way for a support player to join the team but instead they get another starplayer (again) then obviously they need to convert one of their other players to dedicated support player. In my mind the best possible core that FaZe could keep right now are Olof, Niko and Guardian since they seem to be the most consistent so far and that is saying a lot. Out of that core Olof would be the best pick for dedicated support player as for starplayers there are a few out there that have proven themselves so if they could pick anyone then it would be most likely they would go for one of these players: S1mple, Scream, Zywoo, Sunny, Ropz, EspiRanTo, XANTARES. Or they could get "fresh blood" from FPL, a young guy like d0cc or someone like that. Could be a success story like it was with Ropz, Zywoo, Brollan etc. but then again I don't know if FaZe would take such a risk.
This of course would be best case scenario if they could get any other player other than any of the Astralis players and honestly I think the people mentioned would think about taking this offer just to compete with Astralis on a consistent basis. But contracts make this entire endeavor a lot harder to accomplish. There is a very small chance Fnatic let KRIMZ go, MiBR let Cold or YNK go etc. and that they would have to wait out their contracts.