Debatable competitive viability aside, I really like the design concept Mega Aggron seems to exude. Most of the Megas are kind of expanding or adding onto the Mon's design and Physiology: Altaria gets fluffier, Zard-X gets all those spikes and claws, Swampert bulks up, Gardevoir's "Dress" plumes out, etc.
Aggron appearance definitely changes, but it feels more like "refining", both literally and aesthetically. Its design loses most of the black, becoming pure metal across like they refined the iron in its body to pure Steel, which the type reflects by dropping the Rock typing, and the ability filter, as pure Steel is harder to score/damage than Iron tends to be.