I don't think a pick/ban phase is viable with only 21 heroes personally.
Whoops, sorry, meant just a hero ban phase. Like I said though, I don't really know what the competitive community is going to want from Overwatch. I'd personally be a bit disappointed if it came down to banning heroes (unless they're super imba) as a huge amount of the depth and fun comes from the vast hero pool.
I think a 1 hero limit is required and if heroes get left in the dust then blizzard will need to buff them back into it just like dota did, over 100 different heroes played at this Manilla comp.
1 Hero limit could be accompanied by a pick and ban scenario, for example banning Winstons / Lucio / Mercy / McCree's on certain maps
Lots of ways to bring diversity, just need to think differently
A single hero limit with a short hero pick/ban phase for each map was what I assumed the competitive community would end up going towards last year when I first had the chance to play it. But that said I don't even have enough time to play the game at the moment, let alone keep up with the competitive community, so I have no clue if that is something the community is even interested in.