It struck me that your 4 categories have an ascending scale of physical contact with the Domme (I mentioned this is my reply to swordfish) I would therefore propose that FinDoms along with all online only Dommes be assigned category 0 on your list as they afford even less contact than type 1 on your list being essentially incorporeal.

Other meaningful categories would be most meaningful if the category tends to have an identity. I see that Aspx has brought up Switches and that is a very good example. I would add 2 more. The first is Disciplinarians. Quite a few of them do not see themselves as being dominatrices at all. This goes hand in hand with their disingenuous tendency to avoid acknowledging the eroticism of their activities as much as possible.

The other meangful category is the FTT practitioners. This is largely because many subs and more than a few Dommes tend to look down ob them as being the Untouchable caste of FemDom.