If we're sharing balance suggestions, here's some of my short-sighted ideas:
Ana definitely is in the most need of being addressed. Her healing-rate is really fast, long range and works best on larger characters like tanks. Her grenade also has AOE burst heals to her allies and herself, and even denies heals for others and damages them, making it really versatile. The nano boost is just the cherry on the cake. I'm not sure how you could change her though to stop her from being the sultana of tanks without changing the very essence of the character. As someone that plays Support a lot, the only situation where I won't pick Ana is when someone else has already picked her. It's obviously a problem when one healer excels in virtually all situations, since it trivialises the existence of the others. Perhaps Blizzard needs to figure out what they want "her thing" to be, and just home in on only that. A change I'd propose is to make her biotic grenade come straight out of her own HP. If you use it, 60HP is removed (or however much is still left without her killing herself, and use that to burst-heal/damage the rest. It would take away her self-healing, and add a risk-reward element to her ability, and give her a dope bloodmagic-esque vibe.
On the subject of DVa, I knew her steady stream of buffs were trouble. She was slowly getting out of hand, which aided the rise of Zarya for a while. When they brought Zarya down a peg however, they not only kept the DVa with her buffs, but also slowly continued to buff her further. They clearly wanted her to become relevant, and now that she is, she feels insufferable to play against. Her biggest downside used to be that she had trouble finishing characters off by herself, which was a more than acceptable thing for a tank in a team-based game. Now that she can finish people off by herself, she's in a place where she's not going to go down if she has a healer with her. Every engagement with a DVa turn in slow wars of attrition. If it were up to me, I'd bring her health back down, or make her reload her guns.
Soldier's high damage output is pretty formidable at the moment, but I suppose this is fitting for an attacker role. I'd still marginally bring down his damage output, and completely re-work how his healing works. I'd make make it so his biotic field has a finite amount of health to draw from. If more people want to make use of it, make them share from the same finite pool, like a Half Life health station.
Mercy just straight up needs a new utility feature added to her kit. I'm not sure what, but I really don't like where she's at right now. Right now she is very actively involved in something she doesn't really do especially well. Her kit also encourages her to bail on her team during risky fights, so she can hide and wait for a rez. This self-fulfilling prophecy dynamic seems counter-intuitive to her role and characterisation. Much like Ana and Soldier, I'd once again tweak with the healing mechanics. Since she is most useful when she is attached to a player like a leech, maybe her mechanics should reflect that. Harming a Mercy that is attached to a player, should perhaps sap the HP of the person she's attached to first. It gives her more incentive to stay in fights, but also makes the switch between healing and boosting more valuable.
Finally, Roadhog is a hard character to balance, much like Ana, since his very essence is part of why he can be an issue. He's supposed to be a slow tank with no mobility options, but that can pick off vulnerable characters. He does this great, except when his hook gets really messy. It's essentially the same issue people had with Hanzo months ago. Either the hitboxes are weird, or netcode makes them look deceptive, but there's nothing more unfair feeling than when a hook that misses you still manages to latch on, walls be damned. It looks as broken as it feels. Essentially the hook needs to learn how to let go better. It feels like an obvious suggestion, but I guess it's the only issue I have against the fella.
