Yes, he could.
Sure the MCU has shown that he has become a friend/brother/ally....but whose to say that cant or wont change?
In my own ideas for a sequel, M'Baku and many in Wakanda have began to challenge Tchalla's rule due to:
1) During the events between Infinity War & Endgame- which was a 5 YEAR GAP, there was no mention of Wakanda finding or having a new King or Black Panther to assume leadership of Wakanda.
Since that is the case, Who led Wakanda?! Okoye? M'Baku? Did Tchalla's mother, Ramonda (who is Queen Mother), assume leadership? The MCU showed that none of these indivduals were affected by Thanos Snap.
So.... M'Baku could have easily become leader of Wakanda after perhaps winning Ritual Combat.
Also Wakanda, as shown in comics, has the ability to rule themselves without a King/Black Panther. Perhaps after 5 YEARS without a King/BP has led them to believe they dont need a King.
Maybe M'Baku is now the voice for the people and having a King "force" his way back in power conflicts with that. Maybe M'Baku, after leading Wakanda for 5 Years, does not want to relinquish his power?!
2) Perhaps M'Baku and many others, after the many distresses Wakanda has faced after opening up to the outside world, have now become bitter and weary towards the outside world- and such conflicts with Tchalla's message and goal of trying to now help the outside world.
So no doubt these conflicting ideas will cause tensions to raise, resulting in perhaps a war between The Jabari Tribe & The Main Wakandan Throne- and thus making M'Baku a bigger villain.
Point is...there is nothing that demands for M'Baku to stay an ally. He may actually become Wakanda's version of Loki- we dont know.
M'Baku's relationship with Tchalla could easily change after all Wakanda has faced.