what if the collar above the [nipping loop] were made using a girth hitch/bull hitch/clove hitch around the standing part, before being tucked through the nipping loop?
... just [tie] an overhand around the SP in the place of the collar and tucking the working end down through the nip as usual.
I like all those solutions, and I have to admit that, although they might seem obvious to many people, they have never not crossed my mind as improvements of the bowline
Probably because, in the cooperation of nipping loop-collar pair that characterizes the bowline , I tend to see the nipping loop as the primary structure, that needs/can/should be a little more complex, ( than the common nipping loop of the common or Eskimo bowline ) , and the collar as the secondary structure, that needs/can/should remain as simple as possible. The loops that belong to the bowline family secure the tail by the nipping action of their nipping loop, and it is natural for one to think that he can improve the bowline by improving this nipping action, i.e., by adopting a more complex nipping loop. Now, a more complex collar does not need the nipping loop at all - or, to be more precise, it does not need the nipping loop as a nipping structure, but only as an obstacle on the standing part, that will prevent the collar to slip alongside the line. Personally, I believe that this type of knots might well be very secure, indeed, but they do not follow the spirit
of the bowline... so I would not define them as fixed end-of-line loops that belong in the bowline family.