While trying to get to the end of HHP to unlock stuff, I too felt the burn out. I felt like there were too many items left on repeat.Somewhat, but not for the same reasons. I just get burned out having to design all these houses... I am still not done playing HHP as much as I intended to and the burnout is the main issue.
But with that said, I always get burnout from playing Animal Crossing. I've never finished any of my towns or my island because of this.
I think the problem with HHP is that there are too many villagers to go through and some of what they ask for over lap.
Personally I would have been fine picking villagers from an application pile to decorate houses for on the small island with the facilities in it,.. but at that point they might as well just waited and do a new full game.
HHP I feel was them testing waters for some ideas they want for the next AC game.