I think a parallel twin with non standard timing would work great. The Triumph Scrambler, Thruxton, etc use these kind of engines to great effect.
I know there have been RE's and British bikes with V-Twins, but frankly there are so many Vtwins out there that I don't think its a great direction for RE. But I did love the designs and its a fascinating idea.
KTM has the 690 Duke, which is a well respected single cylinder motorcycle.
I know for lots of folks I talk to the fact that the RE UCE bikes can't go over 80 with the stock equipment is a problem for them. It pushes some people to Triumph, or other motorcycles.
Yet I understand why 500 is the biggest bike for RE. They build for their home market first. But perhaps with Triumph and Harley Davidson entering India, maybe RE will have a 700cc parallel twin or something in the pipeline.