Decreased summoning range (and decreased summoned figures or cards per turn) is an option. A limitation to summoning that applies in early rounds (first 3-6 turns?) but not in the later rounds is another option. I think the simplest tweak to start would be to have the starting spell hand be empty.
The
@
chas
solution (which appears to work well) is to require each player to have a Planeswalker and increase the point limit in kind. That is an option that would require no changes to AotP but would have a big effect on HS players. Chas also plays where HS figures are immune to sorceries (rather than just enchantments, which is how I have tried it so far).
Another idea I have had is simply change the map pool to feature mixed start zones (see A Stone's Throw or the two latest additions to my own map thread). Or play Heat of Battle.
All these changes are built on the assumption that the thing that makes AotP armies "too good" is rapid board control and auto wound abilities (primarily via spells).
I'm not sure any of these address the Eldrazi Ruiner and Avacyn, which are the two most troubling figures for HS armies, in my opinion.