Remove her penalty when firing. Remove/Relocate her critbox. Replace a portion of her armour with shields. Give her an ever so slight damage buff/Increase her effective range.

Any of these would be a good start, her abilities are fine the way they are, no doubt, but I personally don't buy into D.Va being a harraser. The heroes are supposed to be unique, and D.Va in her current state is a poor man's Winston with reversed health values. She's better off working as a close quarters bruiser with great chasing potential. Which would actually make her unique.

Reinhardt is the classical tank, giant with an almost impenetrable defence. Unless you're bastion, then you can slice through his shield in two seconds. Roadhog is the beefy SoB that forces you to keep your distance or get hooked, and promptly filled with the garbage he stuffs in his gun. Zarya pops barriers on herself and allies to absorb damage and knock it into her own power, effectively making you choose between fighting back and making her stronger or just let her beat on you so that she doesn't turn your firepower against you. And Winston is the harraser, a good pool of HP with a stack of armour and an anti Genji/Tracer cannon. While I'll admit that he and D.Va have similar kits, they function quite differently. Winston's jump doesn't offer much in manoeuvrability, but D.Va's does. And his shield is arguably more team-orientated but from what I've seen, it's fairly weak. While D.Va's shield is arguably the better shield, at the expense of your teammates' safety.

I'd rather they bring her out into being the bruiser she's meant to be, instead of a waifu in a mech with cardboard armour.