While the expansion pack for Heroes Of Might And Magic V is just a visual rehash of the original, The Asian Dynasties (TAD) shows just how add-ons can be done, bursting with rich gaming pleasure and significantly enhancing the core AOE III engine.
There's more going on than I usefully have room to list here, but the handover to a new development team (Big Huge Games in this case) has allowed the series to be treated to a fresh, yet reverential, set of eyes, ears and hands.
Thus Wonders make a return, spicing up your end game options, the clever deck system gets a few race-based twists that make you rethink their usage in a way best described as 'completely' and a hundred other smaller tweaks, tinkers and amendments make it play like an almost totally new game. Plus it still looks beautiful.

TAD is pricey for an add-on pack, but if you can stomach the cost, there's plenty of succulent, meaty goodness to be had from TAD. Plus it will big up your brain, and that can't be a bad thing for a pony.