I can't speak to roster share, but one of the major issues with player separation is that a) they have to make ratings for guys from NHL to WHL and Europe, so they can't just make those players like 25 overalls, it wouldn't work playing those games and b) they don't have enough ratings to really give players different strengths/weaknesses. As for a), someone had a great idea to just add a "league weight" qualifier so that if a guy is a 99 in the WHL he's a stud but he'd be just a good player in the AHL, etc. As for b), I'd like to see them add in more things to give guys more a range.
This obviously would take a ton of time and a general re-working of their entire roster system so I'm not surprised it hasn't been addressed and no developer wants to say, "we're not re-rating every player because it would change the way the game plays substantially." Not to mention the niche that would really use roster share is probably pretty small in the grand scheme of things and one other thing - they can't hype up their legend teams if people can just create their own. I disagree with their disregard for these features but can understand the logic.
Roster share would be great but would be difficult with all the leagues and league rules in place.
I didn't say it is a masterpiece and it's not, and they've clearly invested in the offline experience of BeAGM mode even if you think they haven't done enough. I wish they made it more of a priority but unfortunately, this is where we're at.