Having not played the game myself I don't know how strong these points are, but I don't think the multiple hero thing is a problem. The game's been designed not only to have the option to change heroes mid game, but to encourage it to adapt to your opponents' strategies. Like, I heard that getting all 6 people as Winston was super strong a while ago, but it turns out that it was countered by all 6 opponents switching to Reaper and taking advantage of his ability to shred through tanks. Likewise, if you're against a few too many Bastions, send some Tracers in there to bomb 'em.
Also I want multiple heroes to stay because I've been waiting to play as Tracer for like a year now and if I can't play her on release because other chumps keep picking her I'm gonna be friggin' furious.
I'm also surprised you don't mention Zarya in your "desperately needs balancing" list. Roadhog and Tracer have some pretty outstanding weaknesses (Roadhog having only health and no armor, and Tracer falling over if she takes just a couple of headshots, for example), but from what I've heard Zarya has very little going against her. A tank with shields for herself and allies that reduce damage, and if she does that right gets rewarded by being able to deal huge damage on her own. And citation needed on this one, but don't her shields prevent crowd control effects like Roadhog's hook and Mei's slows? If so, there's another point in the favour of the games balance based around counterpicking.