I can't imagine a Lumineth release before pretty much every other faction has had some substantial attention. They've left nice spaces for future development - Tyrion, River, Zenith, etc - as you've noted. But I think it *should* be 3 years before Lumineth receive a new wave, given the state of other armies. Even if gw decided to give more based on sales, that sale data and the development saga places those releases at 2-3 years away.
It might be of course that there never is a third wave, and the army receives only thay kind of one-off characters that armies kind of get as a tide you by update in both 40k and AoS. It could be therefore a decade, or two editions' worth of other factions, before any kind of real update
Anyway, as the third or fourth-most modern range, LRL are in a very nice place, mini-wise. They won't always be as strong, and presumably will drop through the edition(s)'(s) cycle, but that's ok and the way of things too