I would constantly set goals for myself. The game is starting to get a pinch boring but I'm trying to get 10 million with turnips, it will help you a lot.
make every flower in your town blue roses, have all the furniture sets in your house in total, have a complete catalogue, set up museum displays, get dream villagers, re-construct the island. Make a billion bells and then give it all away.
Make a really beautiful town... in my case though I can't wait till this game gets more boring for me. xD; Being active on the forums makes me even more addicted admittedly though. My school started today so I'm trying to not play as much but it's hard. >.< So many plans in mind.
After I finished my loan, I tried to get the top score for the HHA theme challenge. I've done that, so now I'm clearing out my rooms of complete sets and making them the way I want to. Also constant landscaping, getting dream villagers, and collecting furniture. If it gets boring, I'll just leave it alone for a few days and come back. That always works for me.
I never thought about what I would do after I paid off my own loan. I agree with the people who said to start a new character (in the same village) and start to pay off their house and collect furniture for them. I would try to collect really hard to get furniture/expensive furniture for the new character. Don't forget to fill up your second level Museum with interesting themes! AMG I just though of something, totally off topic but I think I will request a 2nd level museum contest for November.
Back on topic, try to grow hybrids and fill your town with them! You could also (if you don't already) start visiting a lot of dream towns and get ideas to make your own special dream town.