To give you an idea of the power level associated with increasing the size of a weapon: there is an epic destiny which has a feature of making the character taller/heavier, giving them +1 reach, and allowing them to wield larger weapons. Given that, I couldn't imagine there being anything else that would allow an increase of weapon size besides an epic destiny for quite some time. If a paragon path or even a feat from epic level were to allow that, there would need to be some way to prevent it from stacking with the Eternal Defender.
Any kind of giant like race could benefit from a race specific epic destiny to become larger, although truthfully with the way 4th edition works the only advantage to being large is the weapon size. One could easily show that the increase of STR/loss of DEX to get large in 3.5 was a reasonable tradeoff (to say nothing of reach!), but in this edition, being 2x2 is effectively a liability, and a big one at that. Perhaps if there was an allowance to increase the weapon size it could be balanced against becoming 2x2 (without reach, ofc), but these are all some pretty heavy variables that could easily spiral out of control. I think wizards will be slow to start changing the size of PCs.