An expansion or a sequel? I'd pick the first, but to be frank, I'd rather they tasked a small team with rebalancing and reworking existing features. Over at Stellaris, there was exactly such an initiative, called the Custodian team, and the idea worked out splendidly. Of course, Frontier doing the same for an MMO would be much more risky, because compared to a (primarily) single player game, any significant changes are going to upset more people, regardless of how well-designed the changes are. It just seems to go with the genre.
But hey, the vague roadmap we have does include a rework of some key feature, so we are going to see what the game design team is capable of, and how said rework (whatever it'll be) will be received. On one end, maybe it'll be so bad that players would rather have the developers not do the same for other areas of the game; on the other end, maybe it'll be so good that players will want the developers to do the same for other areas too.
If I were Frontier, I'd prioritize "deepen, rework, rebalance" over "expand", at least for the time being. Right now, that might even draw in or draw back more players than another expansion would: after seeing how Odyssey's launch went, players would be a lot more cautious about buying another expansion, even if the ideas of said expansion sounded good to them. Instead, putting out some good updates first could go a decent way.