Make lvl 1 Pop Pop and overhead.
Hear me out.
One of the biggest issues Harley has (besides her shit trait) is no one has a fear of ducking against her. We're all in pretty much full agreement that f2 is lackluster and basically only catches sleeping opponents and f1,3 is hard as shit to land without getting poked out of. What Harley needs is something that hits overhead midstring rather than first hit (not counting f1,3).
How this helps:
As mentioned, there is little to no fear of Harley's overheads in their current state. This allows Harley to follow up strings with an overhead to force opponents to respect her mixups.
"But OMG that's so OP! Harley would be broken!"
Not at all. A lot of the cast have midstring overheads, overhead/low special mixups, overhead to low strings, or even 50/50 mixups off of normals (Killer Frost anyone?). For Harley to make use of this, she would have to MB the Pop Pop. So add the fact that it would cost meter for a mixup and the fact that Pop Pop has a chance to fail (yes, even at lvl 1), that's already paying more than half the cast for the same results. Generally speaking, you can't do much with Pop Pop right now and it only combos off of heavy staggering hits like f1.
As of right now, most of Harley's meter usage is going toward MB b3 with the occasional MB Cupcakes or Play Doctor. This gives her added depth and makes meter management of more importance to her. Additionally, this makes lesser used strings more useful and gives her more mixup/block string options, but at the cost of meter. Added fear of the Pop Pop overhead also allows for f2 to open people up more.
Ideally, you'd be using things like:
1,1,2(blocked) xx MB Pop Pop/TS or b2,2
2B3(blocked) xx MB Pop Pop/TS
2U3(blocked) xx MP Pop Pop/TS
1,1,2 would have much greater mixup potential since it is + on block, but not so much + that it would lead to something like 1,1,2 xx MB Pop Pop "block infinite" since Pop Pop does have some slow start up (but not as bad as f2), allowing the opponent some room to react without putting Harley in a compromising position. Additionally, strings like 2B3 and 2U3 would be able to be made safe (at the cost of meter), but wouldn't be too OP as you would have to commit to your mixup should you choose to TS instead which followup options are punishable.
In essence, this gives Harley's Risk vs Cost vs Reward more reward for her efforts without making her over the top. (and making up for a shitty trait....).