I think the loss of ambushers and the unreliability of invaders hurts newer versions of the game in succession forts, too. Getting enough interesting enemies to show up in the vanilla game is a big problem.
I believe this is a big turn off for a lot of people, yes.
It's anecdotal, and my last fort was a simple community fort and not a succession fort, but I'm convinced the lack of invaders severely curtailed interest in it.
To be fair, as a community fort that was probably exacerbated by the fact that players owned individual characters and not the fort. So, if they had a soldier and no invasions ever happened, they get bored and lose interest. By the time something
did finally attack (a werecreature), everybody had quit already.
For succession forts, invasions are still an important source of spice and variety. Even with megaprojects or screwing over previous overseers, the game just becomes kind of routine these days without them. That's kind of related to general burnout, where people have either seen or been part of the same stuff for a long time. The next game release promises to have player instigated attacks, so that might help alleviate some of the boredom.
But, that all really probably belonged in the other thread about the decline in interest. As for ideas, I don't have much, I'm afraid. I'm not interested in playing in succession games in general and prefer to stick to story / community forts where I can craft things carefully without worrying about whoever takes over next doing something bizarre.
One idea I've kicked around in the back of my mind was to try to solve the boredom-by-no-invasions problem by running a pure military fort, where literally everyone that could be drafted was. It might even be amenable to succession games if you wrote it up as base commanders being reassigned.
If you could find somewhere appropriate to embark, such a setup might be mildly interesting, but short of plopping down next to a necromancer tower I'm not sure anything gives reliable invasions anymore. That, and I'm sure military forts like that have been done plenty of times, so it's probably not an original idea at all.
Alternatively, I've been considering posting a story fort of some kind without public players, just as something to read. Maybe even a hermit fort, since I've never tried that before.