If you want to stay 3 channel your choices are Ember 2, Vapor, J-3 and.....well......okay, that's it.
If you move to the Ember or Vapor you're not really building your skills. They're fun airplanes but slower and easier to fly in general than your much faster Champ. Unless there's something that really appeals to you about the character of the planes, I think there's a possibility you might not be happy with your choice.
The J-3 Cub is a great looking plane, a little faster perhaps, but pretty identical in manners with the Champ. You'd probably be happy with the purchase, but I question whether it would teach you a lot more about flying. As a confidence builder, I'd recommend that one IF you are settled on a three channel plane.
If you're not satisfied with those choices it's time to move to four channels. Then you have the difficulty index already excellently covered by Saddlebum. I would say the T-28 looks like the best choice.
I'm in the same position and the T-28 looks like my path to 4 channel too. Keep in mind that the parkflier T-28 will tend to be better fllying, easier to fly and handle better in wind than the micro. But I get the idea that you, like me, are hooked on the micro path. No problem with that as long as you realize the limitations.
Onward through the fog!