To be honest, GW keeping space marines as cis male warrior monks is probably the safer option.As far as canon goes the answer is a zero percent chance as long as GW stands firmly bolted to the idea of all space marines being (inherently cis) male warrior-monks. We apparently had the chance to get female space marines back in the day but the sculpts came out looking like shit so they scrapped them and then just never bothered again. As long as the writers are wedded to the idea and/or feel obligated to certain loud parts of the fandom, that's just not going to change.
I spent an hour or so trying to come up with a good idea for a female space marine, but everything I could think of ended up being too problematic.