I've sometimes wondered about this too, if it would be possible/a good idea.
A Back Smash could be a cool addition. If the input is as accessible as you describe it (especially if you're using a C-stick), then these could work and add a bit more depth to gameplay. It'd probably rile up competitive players (maybe in a good way, maybe not) as it'd be really different, but if it didn't make the game any less competitive (I'm going to suppose it wouldn't), then it'd likely be embraced by the community. But it could change the metagame a fair bit, who knows.
If Sakurai and his team decided to include Back Smashes into the game, I don't think they'd have trouble coming up with things. Some could be simple reverses of the Forward Smash at the last moment to trick opponents, kind of like the Falcon Punch. Others could be more legitimate in uniqueness, although I can't think of anything that creative off the top of my head. The only thing I can think of now is Charizard using his tail or something.
Basically, my thoughts are:
- It's probably easily possible to implement, with the right mechanics
- It'd probably take time for some people to get used to it
- Competitive players will probably like it if it doesn't diminish the competitive nature of the game, which is probably debatable
- Nothing would be holding Sakurai's team back from coming up with ideas for Back Smashes
We're probably never going to see this idea happen in an official Smash game, but I think the idea has an interesting appeal to it.
...Although I think the Down Smash has played this role effectively enough that we don't need a Back Smash very badly.