How about something for the bottom. A school of 6 cories or so would look great and get on fine with the occupants you have already. They are lovely little creatures, very entertaining and peaceful. Just make sure that the gravel is not sharp, as that could damage their barbels: they need either small smooth gravel or (even better) sand. Another option might be a bristlenose plec. I would go for the cories in the first place, as they would add a lot of activity to the tank.
If my maths are ok (don't trust them, check out my signature!), this would still leave you room for another small school: how about harlequin rasboras? Or else, you could up the number of the neons- they look very impressive in big groups.
Just don't add all the fish at once.
