Not sure, I find it difficult to judge his ceiling, because of how poor of a learning environment he's had since moving from Monaco. These are quite character building years and progress is difficult to judge somewhere where you don't have a huge challenge or competition in the side or experienced men to get some lessons from. What can Cavani or Thiago Silva possibly teach him? How to get injured? That said he definitely IS at a level that allows him to be in the conversation for future BDO down the line. I think he has it in him to be a serious scorer, at the level of Ibra at the very least, possibly a Suarez, in any league.
I find it most tricky to assess his potential relationship with Zidane at Real. On one hand, if it happens, Zidane has a spectacular record working with players he personally rates. Not one person really fails him when he backs them - maybe Isco and Benzema who's have been up and down like whore's drawers, but that is also because they're not some outstanding physical specimens. But young guys he actually helps mold that have a fairly natural gift? Varane, Casemiro, now Fede and Mendy.. If Mbappe has that same link and works out how to be useful to a team that almost never gets hectares of space then he will be a force for up to 5 years in a Madrid shirt and for sure could be the best ever French forward. At least statistically, even if it's not in Madrid he ends up in 2 years.
On the other hand, if the team aren't up to it after a cycle of success, I don't think Mbappe is the guy who would break that spiral like GOATs tend to.