I think it would be difficult to do Black Panther without either Black Panther or Shuri.
I can see a couple of options for BP 2 myself.
1) Past Wakanda: Its a movie about an earlier time maybe a generation or two back. Or we go all the way back to the first Black Panther or something. I think this is a unlikely option just because I think the MCU is focused on moving the plot forward in the movies, leaving the TV shows to fill in various gaps. So I could see this as a TV option, but less likely as a movie.
2) Blip Wakanda: We start with a Black Panther funeral scene, and of course the audience assumes its modern day. But then we find out that no, its 5 years ago, when the Wakandans thought their king was dead after the blip, and had the funeral then. The movie focuses on Okoye and maybe Shuri. Shuri doesn't feel worthy to be King (she is in theory the next in line I believe), and so goes into hiding rather than vying for succession. Okoye has to put on the mantle of black panther (extremely reluctantly of course) while Shuri advises and helps her with the tech (maintaining the "Q" to her James Bond). Obviously Okoye gives up the mantle immediately upon the return of her king, but this gives us the option to make her the next Black Panther when T'Challa dies for the "second" time, in whatever way the MCU handles that.
Obviously the blip could cause a million problems you could use as your plot driver, take your pick.
I know a lot of people want Shuri in the suit, but I don't see it. Shuri is a scientist at heart, and she plays the supporting "Q" type role very well. Okoye is the fighter, she has the heart of Black Panther to me, so I feel like it would work much better with her in the suit and Shuri supporting with gadgets and tech.
Of course nothing is ideal compared to having Boseman in the suit, but its a viable way to go.