I can't see more tiers coming to Mosaic, they'd have to offer too much to make the additional level worth getting, or it would just be a straight give-away to those who are already spending above the $5k level. I wouldn't be surprised to see the qualifications pushed up though, maybe to $6000 or $7500 at some point.
JetBlue already has the best First class lie-flat product for transcons, they can compete with product alone, and don't need to lure people into flying them by way of FF program as a side benefit the way the legacies do.
All that said, I wouldn't mind a few little perks like maybe Mosaics being served first in Mint. I've waited a _long_ time for meals in a full Mint cabin where I know there's only one or two Mosaics. Not a big deal. Definitely 1st-world problem, but would be a nice gesture. Other than that, the best thing that could happen to Mosaic is for the airline to actually expand to more usable routes for those of us on the west coast.