erin richards is talented af and could easily pull it off (I even think she could do the typical harley voice/accent but there’s no way to know for sure, plenty have struggled to capture it)
we’ve seen her interact with jerome so we know any future joker/harley scenes could be good
she interacted with selina before she (barbara) became a villain so it would be interesting to see them interact now or in the future if she becomes harley (and selina becomes catwoman)
there’s going to be big villain team ups in the rest of season 3 so there’s a chance we’ll see her interact with ivy. with ivy’s (mental) age I doubt we’ll get a romantic or sexual ivy/harley but we could get a chance to see that friendship before babs becomes harley
barbara is ambitious and she’s never come across as having a low intelligence. although I doubt we’ll see her become a psychiatrist there’s nothing to say she couldn’t if she tried, but assuming she has some kind of criminal record out there she might not be able to practice anyway.
and there’s many more things I could list, but the point of this post isn’t to try and convince you that she’s going to be harley or that it’s a great decision, but that it’s also not random. she’s a decent choice.
would she be the perfect harley? as much as I love her, no, probably not.
ideally she would make a good prototype. kind of like how tabitha (who, yes, I know is tigress) has some catwoman qualities and is going to potentially train selina next season. babs could very well be the one to introduce harley to the criminal world. this could also work with lee too, have the build up where it seems like babs and/or lee are going to become harley only for it to be revealed that they’re not, but instead are a huge step for the real harleen quinzel to become harley.
the other point of this post (before it got long) is that really the only or main reason people should have as to why barbara shouldn’t be harley is:
she deserves a decent story line outside of jim (and other similar characters) and she shouldn’t have to become a completely different person and/or famous comic book character just for that to happen.