Ok, unpopular opinion time I guess. Harley's a pretty good zoner, when she can actually zone. Reactive zoning isn't really her style, though. The problem is, she can't do that against most people with a projectile.
What she needs buffed in that department is counterzoning, but it's hard to do that without then making all her good matchups worse. We don't want to fix her bad matchups just to make life hell for the characters that already struggle against her.
I think the way to buff Harley to do better in matchups where she gets outzoned is to make air guns much more threatening. If she got more off that, or god forbid they took the auto block off MB (massive overbuff right there, but I'd love it), that would even out things quite a bit, and if they made it more punishable, then it hopefully would even out for the matchups she currently does well in. It'd make her a bit better up close too.
Also, one of the buffs I think is most important is to make upclose gunshot pressure more consistent across the cast. Having your primary pressure tools flat out not work on multiple characters and be unreliable against many more is just infuriating.