what do you mean by custom game system in pbp games? i'm not sure i understand. so far as i've seen, all pbp's use whatever game system they choose, just like tabletop gamer groups. some use D&D, others Palladium, others FATE or FUDGE or GURPS... please explain?
for example: what i would want in a pbp software, is a light-weight forum which can be hosted on a windows, mac, or unix server as well as on a windows, mac, or unix personal computer that supports pictures and dice rolls of all sorts, that can be posted to via sms, mms, email, or direct forum posts; and is thus accessible both for posting and viewing from android, ios, mac, windows, and unix platforms. it should also be able to send update notifications to email, sms, mms, and upon login from a computer.