I didn’t read everything here (so it may have already been mentioned) and I don’t speak for the devs, so please take this with a grain of salt.
However, one thing PoE cannot do easily is add additional base classes. Now, I imagine that would likely be as large an investment as EHG could possibly make, but it is theoretically feasible, and is another axis for growth that competitors like PoE would have a difficult time directly reacting to.
As an aside - we’re getting more ascendencies in poe2, but more mastery classes on existing bases are also theoretically (I hope) things that can happen in LE when the time comes.
Additional means of keeping league-type mechanics fresh would be to rotate which base classes (assuming there were more) or masteries were available within a given league. Restriction breeds creativity and all that.
One means of keeping leagues fresh that I would NOT recommend would be league-specific timeline / story content. To clarify, I mean content that would expire at the end of the league (persistent content would obviously be good). FOMO is a terrible way to retain player base, causes very toxic interactions in the community, and should generally be avoided.
Food for thought, hope it helps.