Let me guess, time travel.
Not necessarily...kinda dismissive actually. I'm in the same boat as
@jumpman, I feel like I've "finished" my island too and I don't time travel (on this particular island - the one I play my day-to-day on).
Like, to me at least, collecting more furniture and DIY's isn't what keeps the game feeling alive to me, it's just something that happens a few times a day. It was unlocking new buildings (which, if you've played since release, you'll have unlocked everything by now that you can currently unlock), raising "friendship" with villagers, unlocking all the rooms of my house, and designing my island how I like it. I've done those things. I could introduce new villagers slowly, but that's boring - when you've been through it with one Cranky villager, you've been through it with all of them, and sure I could redesign my town (and definitely will eventually), but that's just intentionally looking for "new" stuff to do.
Repeating the same stuff day-in-day-out, to me, is a sign of being finished. New stuff happening and new stuff being available to do is a sign of it being unfinished.
I appreciate the "don't rush it" mindset people have, but...there's not rushing it, and then there's intentionally doing everything unnecessarily slowly just to pretend there's more to do than there is. You can only "not rush it" for so long.