Segregation by sex is not arbitrary, and segration by weight isn't either (even if the actual values of the implementation are).

But, anyhow, the thing you're looking for is the "open" format that already exists in other competitions like chess, where there's an open category and then any specific categories.

Ironically, in dance competitions (specially swing dancing at least), the open category is done the newbies, and higher levels have other more speciallized categories: advanced, invitational, ...