I think the easiest for the next few updates (and also really great to see) would be adding the cars that are missing from FH5 but are in Forza Motorsport and/or were in FH4. These models are all high quality and wouldn’t require a rescan or sm like that. As you already mentioned e.g. the GT86 TRD featured in FM), I would love to see the IS F, Corolla SR5, ofc GT86 TRD version, IS 350 F Sport and the GS350 F Sport back in FH5. Maybe we will also see some FM4 or FM5 cars (Toyota Altezza, Yaris etc., as you might have realised now from my list Toyota/Lexus is my favorite brand), but dunno if a rework would be enough for those or a whole rescan would be needed possibly. This is more likely for FH6 ig, and I want Playground to take their time for FH6 and the cars really.
So in total, the missing cars from Motorsport and Horizon 4 will be easiest to add to the game (btw, this is a nice list: Voting list - missing cars from FH3/FH4 to return in FH5 [Update 2024/10/12: -7 cars])