Like many pilots, "I want it all". Unfortunately nobody's making the low cost fully aerobatic ultralight amphibian bush plane I want , so I guess a few compromises are in order. After the UltraStar I'm flying now only meets two of those criteria (ultralight and low cost). One could put floats on nearly any airplane, "aerobatic" to me means basic loop roll spin kind of stuff, not Pitts Special, and reasonable short/rough field is "bush plane" enough, not Alaskan gravel bars. There have been precioius few aerobatic ultralights (the Quicksilver Super is the only one I know of that was really built for it, and they're pretty rare).
A concept has been forming in my head lately that I can't quite get rid of. Ultralight (for freedom from regulation), pusher (half the fun of an ultralight is the great visibility without any sort of cockpit or windscreen, which makes a tractor design too windy), and biplane (not only because they're cool, but they can have tremendous structural strength and good roll rate). Starting to sound like Mark's biplane, but where he optimized for super light weight and low speed performance, I'll be happy if it just barely meets the 103 limits (using the AC103-7 calculations if not in reality).
The closest thing in appearance would probably be the 1914 Beachey-Eaton "Little Looper", though I'm not interested in building a historical replica:
Unlike the Beachey, however, I would use equal length wings, single bay wire bracing, conventional ailerons, and conventional (tailwheel) landing gear.
Construction... hmmm. I'm currently leaning toward built up wings with doped or Stits fabric like the Kolbs, with a welded steel center structure (engine, seat mount, landing gear) and aluminum tube tail structure. Semi-symmetrical airfoil, gotta find a compromise between low speed lift and inverted flight, perhaps with flaps or flapperons on the upper wing. 2 stroke (depending on what I can afford, 447 or Cuyuna or one of the Simoninis, maybe) and diaphragm carburetor for inverted flight. Hard points for future float attachment (the twin boom tail fits in nicely with that; no acro with the floats though).
Another goal is reasonably easy disassembly for trailering (though not on an everyday basis). I envision removing the forward (cockpit) structure and tail, keeping the wings together and rigged. Room for some camping gear, too, even if it's just a place where I can lash my hiking backpack.
It's not going to happen any time soon as I have far too many non aviation related projects right now competing for time and money... but dreaming and designing costs nothing.
Aviation has made the world a lot smaller, but it's still hard to miss it if you fall.