Here's one way (that really just re-states existing groups):
engine displacement up to .45 (non-schnuerle ported) - or vintage
engine displacement up to .61 (non-schnuerle ported) - SPA
engine displacement up to .61 (anything goes) - BPA
engine displacement .62 - 1.20 This is really the other part of the BPA which I think has a design cut-off of 1996
engine displacement 1.21 and above - currently not part of the BPA, but there are a lot of designs (post 1996) that are no longer considered competitive (but probably are, at least in the lower classes) or there are pilots that would like to compete at a less rigorous level than FAI or AMA

The underlying goal seems to be to provide a competitive outlet for the older pattern designs. Every year more airplanes fall into the 'dated' category. Who knows, 20 years from now there may be a special category just for Oxai airplanes