Oh, keep the lovely little neons. Boost their numbers up to at least seven, even nine or more. A school of neons is so beautiful and fun to watch! You could combine those with clusters of swordtails (try red to enhance the neons' colors) and guppies, add a school of five cories, maybe some glass cats [although I'm not positive that those aren't brackish... pretty sure?]. I would count some kirbinses as a centerpiece because they are so attractive and have wonderful breeding habits.
Alternately, go for a semi-aggressive tank. Angels make for a great centerpiece; large and eye catching but not too demanding and not horrendously fin-nipping if kept with appropriate companions. Go for a school of tiger barbs (make sure you have five or more to keep aggression down! try seven, and they will be lots of fun to watch), and some other fish that can hold their own like some smaller sharks (correct me, fellows, if I'm wrong but I believe redtails are appropriate), maybe even a dwarf puffer? Again, get some good bottom-cleaners like cories, maybe a plec for algae. I always love little critters like ghost or other shirmp and african dwarf frogs, but that's a matter of tase. Clown loaches would be pretty for such a large tank, too.
Of course, one can always do a brackish tank, which is one of my favorites because of a couple of fishes: mollies and bumble bee gobies. I think it'd be lovely to have maybe three trios of mollies, two females to a male; three whites, three blacks, and three dalmation for example. They are very lively and will keep your tank alive. Glassfish are quite delicate and pretty in contrast to the bold mollies' solid colors and a school of them would be appropriate. I hear kirbinses are also appropriate for brackish water, and they are a pet favorite of mine. Get them a little cave to spawn in and you will be able to observe some fun family structure! Guppies, swords, and platies are all appropriate for brackish tanks (yes, I am talking about a lightly brackish setup!). Try either small schools of six, or trios. Finally, get several bumble bees. They are very adorable to watch, and even though they're small they will sometimes be the center of attention! They may even breed if your conditions are right. I'd suggest 3-5 from what I hear around the forum.
Finally, you could consider a saltwater tank. In this area, however, I have absolutely no experience and so leave it to others to explain. (-:
Anyone may feel free to correct my on my above suggestions; I have experience with fishkeeping but it is limited so I readily admit to falibility!
