both make sense to me, but I am glad to see the final format posted here as well.
I can see both metamagic and force as the central figure .... force to balance, and metamagic as the 'source' of all magic (even if its not fully lore appropriate).
From a gameplay element, psychology element, this is perhaps because Metamagic and Force seem to have been combined within the base game.
I mean think about it ... 'Dispel' magic ... seems more like a matter of force, at least as far as getting rid of magical effects (such as buffs and curses), but then, on the other hand, reverting a node back to purity seems like it could be either from Oghma's knowledge (returning it to the source), OR force (cancelling the magics of others).
So in a way, at least the effects that we can see, in Meta 2 you see a spell that could easily come from either force OR metamagic. Perhaps if force spells had been implemented in the base game, and if there was a clear separation between the two, then it would be more easy to simply declare force in the middle as canon ... but as it is I think I could still see either in the middle.
I could see Dimensional as the opposite of Force in a manner that Force tries to bind ppl to contracts while Dimensional tries to evade contracts through loopholes. {going through other dimensions, etc, slip space) ... able to teleport from one planet to the next, even though the laws of physics/laws of magic seem to normally bind people to an agreement of travel through time and space.
I would be interesting in hearing how dimensional is the opposite of enchantment, but I am sure that there is a usable example.
It is an interesting thing ... really.
Metamagic and Dimension both seem like the sources of magic, while Metamagic and Force both seem like the nullification of magic (or, on the case of Meta, perhaps nullification as a byproduct of mastery).
Ceridwen, Goddess of Magic. Would make sense that she is the opposite of Dagda, the god of contracts. Why? Well, assuming that the laws of physics are a giant contract between all inhabitants of creation, then any magic is a breach in that contract. Therefore the struggle between magic and force seem to make most (if not all) magics only a temporary thing in true creation.
So then, what is the source of magic? Well, Metamagic could be considered the mastery of magic while Dimension is the source of magic ... so either could make a proper foil* for Force.
*is that proper use of the word? Opposing sphere in any case.
Dimension, however, seems wholly unable to sit at the center of the universe ... if only because that would be too binding for a sphere such as dimension. No, she would rather sit at the outskirts, waiting for the chance to escape.
Therefore, I can only think of two fates for force. Either as the opposite of Dimensional Sphere or as the central binding 'force' of all magical existence.
Still, I cannot see force as the source, of course, but perhaps just the glue that holds it all together, while Dimensional would rather riddle it with holes, to make creation like swiss cheese to chaotically jump forth and back at will.
Metamagic is certainly a mastery of mana ... perhaps as masters of all the spheres (and thus able to nullify the acts of other gods/spheres) or perhaps as masters of a certain type of magic. If masters of all magic, it would make sense for Metamagic to take the central role. If masters of only certain types of magic, perhaps a foil for mind would be in order.
Still, I can think of only one way to settle this. If Dimensional vs Enchantment makes more sense than Dimensional vs Force, I would gladly concede ... however if this cannot be accurately conveyed, I think I would be happy in continuing to consider Metamagic as the central sphere.