How about a P51 mustang. The low wing profile makes it kind of unstable but would I get the hang of it?
Since you can fly a trainer well and do all the basic aerobatics proficiently, it should be just fine. You have a lot of options, and a warbird is a good choice. Your second plane doesn't necessarily have to do more tricks than your trainer or something new to make it interesting, although you certainly could go that route and get a 3d flyer or something along those lines. Another plane may do all the same stuff(loops, rolls, low passes) as your trainer, but differently. Some planes have a flexible speed envelope, while some are dedicated slow flyers or only handle well at high speeds. Some planes do slow, lazy backflips, while others do a big wide loop. Some roll very gently and gracefully, even slow roll, while others can roll rapidly like a drill bit. Every style and every individual design will vary.
So what will it be? A war bird? A delta? A biplane, triplane, 3d flyer, high alpha flyer, you name it, the choice is yours, and you've come to the right place for information, suggestions, and resources as far as designs and plans and such, as well as the best guidance possible for designing your own custom planes.
If I had narrow your options down to what I think are the three best routes for your skill that will give you the most fun and diversity, it would be a warbird(such as you are already considering), a delta, or a multi-wing plane(a biplane or triplane). Your options when it comes to deltas are enormous, and a solid performer will knife edge, as well as roll much more rapidly than your trainer. Deltas generally have very good speed envelopes, and can fly well at both extremely low and high speeds. Most deltas are also very easy and gentle to land, and capable of high alpha flying, which is a step towards learning 3d. Biplanes and triplanes are very slow and floaty, as well as stable, but have extreme maneuverability and aerobatics.