I agree with everyone else on this one, objectives are the easiest way to keep it fun. Go for that 100% in everything, badges, museum, catalog, etc. But maybe you already completed everything and already did all the super cool decorating and landscaping some towns do.
In that case, I'd suggest get a new cartridge. Kuriboh is right that if you restarted your current save, you'd probably get bored and abandon it. I think it's more fun to have a new game AND an old game, so you can go back to your 'roots' and enjoy what endgame is like. I think it's easy to forget how limiting the early game is and how you'll be forced to either TT or wait it out day by day just to gain the same freedom you have in your original town.
If you start over and get bored, you'll lose EVERYTHING. And for what, an abandoned half-way started new game? Not worth it. Whole new cartridge all the way.
I think another way to stay interested is to seek the social aspect. That's what's working for me right now ! I haven't stopped enjoying the game, still have badges to shoot for, but joining this forum and helping others get things has really restarted my interest in the game ! Seriously, go to the Re-tail section of the forums and browse people's wishlists/buy orders. If you can reasonable obtain things or already have them, just accept ! Making others happy and visiting others can really help.
Basically, if you feel "done" with your town and solo playing, maybe you'll have more fun helping others get to that point.

And hey, this is coming from someone who came back after
four years !