I think mbappe is already a legend, you might have not noticed time has really passed when he excited us at Monaco before moving to PSG and solidifying and developing him self, aside that he has good number of trophies, multiple league one, the French league cup, reached the finals of the UEFA champions league, and even the biggest football trophy the world cup and all of this he was a key and outstanding player in his team, so I can say mbappe has earn him self legendary Rights, not so much for halland although he can become one with his so much talent and physic, but for now I don't think halland is a legend.
I think there is a fine line between any great player, and Messi/Ronaldo duo, they were just the best players in world history for nearly two decades now, that is just unheard of. Was Pele great at his time? Sure but not for long and he wasn't in Europe a lot, was Maradona great? Sure he wasn't for that long neither, was even Ronaldinho who played with Messi was a great player? Of course. But nobody you can think of ever achieved the levels of greatness that Ronaldo and Messi reached, those guys were just in another level and we are very lucky to have them play at the same time and for a long time even in the same league as well, they are just historically the best players ever if you ask me.
Same goes for Mbappe and Haaland type of players, they are great, they are definitely up there with all other greats already, and they will be amazing by the time their career is over, but they will still be under ronaldo/messi level, thats just unreachable for now, we haven't seen anything like that from anyone else.