The next step up from a 28" mini-penny? For me, a 36" hiwheel designed -- and then redesigned* -- by unicycle maker QU-AX (shown here with an eye-glowing window spectator, in an image that does nothing for my vanity):
Tweed Ride. And me without my jodhpurs!)
here's a possible upgrade: a hub with a 1.5:1 transmission! I'm pretty sure a 36" wheel that acts like a 54" wheel is not in my immediate future. Come to think of it, neither (I hope) is a respoking job. But maybe someday.
* Seriously redesigned: much shorter stem, tricky front brake removed, seat hugely improved (and retroized) and matching grips added. At least everything but the grips is a safety improvement and they all materially aid rideability. QU-AX really did right by this product -- and the people who buy it.