Oswald gets jealous pretty easily, mostly because of his insecurities. Usually he will just brush it off for your sake but when someone really is trying to take you away from him, he has his connections to have that person ‘taken care of’ without you knowing.
Ed tends to take things out of proportions and will think someone is hitting on you when they aren’t. Either way, Ed will find out everything about that person and put them through one of his riddle trials and if they get it wrong well, they usually end up losing a hand.
Jerome doesn’t like anyone trying to take what’s his. Later on he will find that person and put on a show making sure they know not to mess with what doesn’t belong to them. Whether or not they make it out alive really depends on how generous he’s feeling that day.
Barbara will pretend like she’s not jealous and then do subtle things to drive them away. Mixing a bad drink or joining in on the conversation uninvited and when they still don’t get the picture she has no problem just kissing in you in front of everyone.
Tabitha knows that you two are loyal to each other but that doesn’t stop her from getting jealous when someone is blatantly flirting with you. Usually she will just come up to you and put her arm around you but if she’s really annoyed then she will just take your hand and drag you away from them.
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.