I would love growing in the game but I doubt it would work as a stand alone class. Being that growing will(and should) take a bit of time as it did in FFXI it would be nearly impossible to balance as far as leveling goes. It would either be near impossible to level or would be leveled far to quickly making it useless as everyone would do it. The only real way it would work is via Botanist.
The obvious way to balance that out is to have a similar activities on the other gathering classes. Fishing could have some kind of different fishing type(perhaps a way of going after very large fish that is different then normal fishing) and while I can't think of anything for Miner I'm sure either someone or SE could.
It just doesn't make sense to make gardening it's own class. Barring adding it as a another thing for BTN they should just add it as a random thing for everyone to do regardless of class like it was in XI.