first off stay with the trainer till you know you are ready for another plane... then stay with it a few more flights
also when you are getting bored of the trainer you can turn that into a great "second plane" by cranking up the control throws, and moving the CG back a 1/4" at a time.
I had a few trainers that were as fun to fly as alot of aerobats.
a few good picks for a (real) second plane would be any of the "sticks" (big stick, sweet stick, ultra stick, etc etc etc etc etc) these will probably last you for the rest of your flying, but still are easy to handle on low rates.
next move into "scale" planes to get a feel for what they do, My first low wing was a Carl Goldberg Super Chipmunk, and I cant say enough good things about this plane. Although it doesnt "float in" like a trainer it still slows down enough for nice landings, has good stall tendencies, and looks great in the air.(esp during a lomcevak)
or you could go for a "trainer" type warbird like the AT-6 or a similar easy flying warbird.
then go to somthing like a P51. I have a spitfire and it is a blast to fly, but you have to be smooth a silk to keep it from snapping out. but if you practice and keep at it the rewards are worth it.
just to let you know I started flying in march of this year and got the spitfire last month so it is doable in a short amount of time. but I spent hours apon hours on the sim purposefully making the airplanes hard to fly to make myself better.