I will soon build my own city using Create a World and I'd like to discuss some fresh ideas for the use of community lots. Somewhere else on this forum we can read about building your own hotel. That is indeed really nice and functional, especially when your sim is just too tired to survive the way back to home. But if you think about it, you can do even more to specialize community lots to new functions.
Here are some of my ideas:
PRISONS (cage your evil sims.)
ZOOS (yes, that will be possible with fishes in pool aquariums and insects in terriums or in special areas that will represent their natural habitat.)
BUTTERFLY GARDEN (those do exist in Holland.)
ELDERLY CARE CENTRE (in case you would like to dump your old fogies, instead of killing them.)
PHOTOGRAPH MUSEUM (not for showing photos, but to add all the objects as art that are needed to fulfill the photographic challenges, which will make fulfilling those challenges a bit more easy.)
FARMS (change a home lot with harvestables growing on it to a community lot, fruit will grow forever.)
SPECIALIZED MUSEUM (gems, metals, space rocks, gnomes, even frozen sims, like they were wax statues, you name it, you build it.)
BAR (to drink alcohol-free nectar, put your best bottles in a rAck when it is a home lot, then change the lot to a community.)
INTERNET CAFE (in the sims 2, sims could play games in a LAN party with multiple computers in one room, not sure if this is included in The Sims 3.)
These are just a few things that came into my mind. I am sure that you can think of more functions, or ways to improve the lots named in my list. Success!