I think they'd do well to not try for another 10 year long epoch of five expansions.
A sweet spot might be three. We've got the preliminary setup, then we can have rising action and stakes, with a cohesive conclusion.
Though I don't share this criticism, a lot of people found Endwalker's story cluttered and unfocused. So it might benefit to wrap it up quicker next time.
Plus, they admit to not having a full idea on where the story was actually going until the early writing stages for Shadowbringers. So they planned Shadowbringers, and a bit of Endwalker together, and started on that path from Stormblood patches onward.
They've talked about how they're sort of seeing how things go with Dawntrail for now, so I think it'd be a perfect opportunity to figure out where this story is ending. Assuming that hourglass key isn't just going to be handed off to G'raha and Y'shtola to be fully reverse engineered in a fade to black, they could use that to keep going with the stories in the future.
Although, I'll be honest, having a "you can explore every shard now" McGuffin sounds downright terrible, as it makes it seem like visiting a new shard will just be as simple as waving a wand, as opposed to the very specific circumstances that brought us to the 1st and 13th. Plus, after the 2nd (I'm not calling it the unlost world, you can't make me) There's only three left that haven't been rejoined. I worry that we'll just blow through them as one zone each next expac, sort of like how the 2nd was two zones this time.