I don't think you can attach any kind of DPS bonus to being the main or off tank, it will inevitably create issues with people not wanting to take the spot that makes them do less damage for basically no reason.
We already had that problem when all it did was mess with your opener and I doubt you're gonna get that out of the playerbase at this point.
The other issue with frontal positionals, depending on what they end up doing, is bosses with any kind of AoE busters or cleaves.