I don’t think Coogler thinks so lowly of T’Challa that he would have him fail just so Okoye and Shuri can save him. Wakanda’s dealt with a civil war and alien invasion back to back so I’m expecting the next film to be all about projecting Wakandan power overseas and how those in Wakanda respond. If Namor plays a role I don’t think it’ll be based around directly invading Wakanda but more contesting their global influence and skirmishes.