A delta wing L-39 could be interesting. Either tailed or tailless.
Another notion was a continuation of the pursuit-biplace series (at least a little bit further). Basically a P-63 with the 2nd seat where the Allison used to be, either with the engine up front or perhaps a twin (using Westland Whirlwind wings as I have those spare). The Curtiss V-1570 should fit in a Peregrine's dimensions. Could be developed into a nice little light attacker later.
Sorry for waffling on.