I think the big things for me:
1. Most custom bras or niche bras are super expensive, big turn off for purchasing one. Women's bras are made in far more mass production, have all the design issues worked out, but are not necessarily ideal fit for men with gyne. If you can't bring it to market in a reasonable price, say $35 or less, sales will be dismal and probably end up losing $ on the venture. Hate to see that. I see sooo many companies come out with a great product but price themselves out of business. Get the price down and they won't be able to make enough, price it too high and they sit on the shelf and never sell hardly any. (OnStar is a great example, good service, but with a re-up rate of <20%, they are killing themselves, considering the 1st year is usually free, and only 20% re-up after that, they have an awful business model)
2. Sports bras tend to make a uniboob look, which isn't realistic. If one has breasts that are of much size at all, they are seen no matter what, so generally a minimizer style is likely best.
3. I have enough problems with fat under my arms, I really want something that will keep that under control somehow.
I don't like extenders, they often itch and then the straps are in the wrong position and the straps fall off the shoulders too easy.
Not a fan of racer back because the straps end up easily being seen, so for me a std bra design if preferred. Not saying racer back isn't comfortable, they are, but just easily seen. Maybe a convertible style would be better, so one could choose. And not pull over. Ever pull over a bra when just out of the shower? Very much a pain. Need band clasps and adjustability.