yeah it would be difficult to make all different ways of gaining exp the same in balance but more playtime is always going to = more exp if that is all you do in that playtime but to have things that are quick to start and some that are quick to finish for the player with short playing time in multiple different ways would be ideal and making it so it rewards you exp and maybe other things like company seals etc. that is in balance with the time you have to put in it would probably be about the best they can do.
They could add something that is only done once or twice a week that is something very short that yields very high rewards exp wise for the very limited amount time you have to put in to it but is also very restricted in participation per weekly or even monthly. I do look forward to the future exp content i just hope for vast variety with all play type and time restrictions in mind that is in balance with time/versus reward and maybe some limited participation high reward stuff.