I think i've been getting tripped up by other germanic languages when thinking about rhotacism. In particular, I've been wondering how it would apply to the masculine singular nominative ending, and the fact is that by the time of gothic this ending has already become -s, and as such, rhotacism just can't apply to it.
I would still like to see some sort of rhotacism take place in gytc, but i don't want to see z and r turn into a single letter. I think what i would like is a sort of runic-style "ʀ" with an indefinite pronunciation until i can come up with something more concrete, so for now i think it's safe to just use the ʒ character, and just wait to decide whether it should be pronounced [r], [ʒ], or, heck, maybe even [ɮ]. (I like ɮ. It's fun to say.)
So we can put down as a solid rule that z > ʒ, but that this has no effect on s from a former z, and we'll decide what it sounds like later on. I would also put this in a chronology where this change occurs after the rule in which voiced consonant clusters become unvoiced (e.g. razda > rasta > rast, not razda > raʒda > ract or rart).
I'm going to start a lexicon and rule page for gytc at http://ling.everywitchway.net/germanic/east/gothic/gutish to keep track of these sorts of changes as they occur.