I also wouldn't want this to be something that ended up "forcing" people into certain formations.

I'd also go so far as to say that the benefits derived from being in any given formation shouldn't be so huge that they make anything else not worth doing. If that happens we'd just be in another "This is OP, This is Too Weak" deal. I think that to combat these situations, especially in the cases of class stacking, making it so that no more than 2 of the same attack type can be "chained" for "synergy" bonuses. So if a polearm WS hits then another polearm WS attempting to add a 3rd would destroy the delicate balance needed to maintain "synergy" and the effect would end, but axe > polearm > h2h > polearm > thm_spell > cng_spell would be fine since no skill type is repeated more than 2x. Of course a maximum number of abilities would have to exist too, maybe, depending on how large the bonus is, you could string up to 16 skills for 10% dmg bonus or +25% effect duration and it might not be to OP.

On dynamic party formations, I think that this is really the idea, certain effects granted from certain positions and distances, there's no "forced" line ups. Ranged and Mage types could array themselves where ever they want just like they do now, but mages being in close proximity to one another would provide distinct advantages. Melee would still mostly want to line up the same way we do now, DDs to the sides/back, Tank in front, they'd just gain the benifit of being able to utilize hit and run attacks from behind the tank when they do need to be in front of the mob, Conal AoE for instance wouldn't hit them so much because the tank would be like a person sized shield for them. 2 melee on the left of the mob would for instance be able to capitalize on one anothers attacks, the first gets the kidney shot and when the mob bends with the hit the other goes for the exposed head shot.

At least that's what my brain sees.