some powers could fit different roles with their theme and aesthetics, why can't we have some powers being able to fit 3 roles?
NO!!! Do you have any idea how long that would take? Not to mention how incredibly broken it would be for years? Had this been implemented from the beginning, yes. Now? Absolutely not.
A player wouldn't have to play all three roles. They could still choose only to be DPS and troller. BUT it would give us more options. I don't see that being an issue. Now CODING for this might be a problem since as you say, it was not an original part of the game. In ESO this is part of the game. Templars are suited to be healers BUT they can easily tank.
How most of the powers look like right now, they feel way too limited to be able to fit additional roles, not to mention how much time it would take to tweak this into the making. Seriously, it's easier to just keep pushing out new powers. Also, it would really undermine the point of creating alts.
There will be a DC Comics based MMO again in the future, but I can (unfortunately) almost guarantee it will suck in comparison to this. Well, at least to us that love this game now.