Someone mentioned in another thread that Blue Marvel will play a role in Monica's next Avengers book, but we'll have to wait and see what exactly that means. It could mean that the relationship will continue, or that they're bringing it to an end. Ewing is scripting, so I'm sure he'll clear up any plot holes if they do go their separate ways.
Monica and T'Challa could work. She's usually in creative limbo, so she could marry, have kids, the whole nine without it affecting continuity or ongoing events. I would caution, though, that they not write her like an understudy to T'Challa in all things, especially the use of her own powers. That smacks of Reed and Sue circa 1960. That won't work in the present.
“True peace is not merely the absence of tension; it is the presence of justice.”
~Dr. Martin Luther King, Jr.
“If I love you, I have to make you conscious of what you don’t see.”
~James Baldwin