Why does there need to be a “unisex” solution? Creating separate bathrooms for transgender people just so they don’t mix with cisgender people is so…Jim Crow.
Unless you mean a unisex solution for everyone, which would make sense. Europe has already done this. When you go to the room marked “Toilet”, you see a bunch of stalls with floor to ceiling doors that lock. Enter one of them, and there’s a toilet and a sink with a mirror. You can do your business, wash your hands, comb your hair, and touch up your makeup all within that stall.
This would not work in places where high turnover is expected, such as theaters, stadiums, airports, etc. In those places, it would make sense for transgender women and cisgender women to use the women’s restroom and for transgender men and cisgender men to use the men’s restroom. Like it’s always been done.
We are a distance out from having European-style toilets as the norm in this country, as you said. Until these become more common, we should do what we’ve always been doing. Oh, and if you see anyone ogling, peeping, groping, or doing anything they shouldn’t — no matter who they are — report them to the proper authorities. Like it’s always been done.