The main goal would be to push ZvZ towards higher eco and longer games, because hive ZvZ is super fun. At the same time, the changes should avoid imbalancing non-mirrors.
1. Put a 8-mineral patch blocking the short rush distance. (Think something like Blue Storm, except the narrow entrance blocked by 8 minerals.) Possibly even put slideable patches on either sides so that the worker scouting distance isn't reduced at all.
This shouldn't change non-ZvZ matchups much, because worker scouting is common, and the scout worker can remove the patch. But in ZvZ, working scouting is unusual at high levels. So, the patch would slow down fast zerglings unless a drone goes ahead to mine the patch first, which might make 12-hatch builds a little safer.
2. Instead of one gas in the main and one in the natural, put no gas in the main and two in the natural. In ZvZ, this would effectively force 12 hatch (or even 10 hatch), because even 12 pool would not get gas fast enough.
As far as openings go, I think the effect on ZvT and ZvP would be minor. FE is typical in those matchups anyway, so apart from slightly delayed gas timing, the matchups shouldn't change much. In TvP, Terran and Protoss would both be forced into FE and get their gas later than they'd like, but maybe not giving an advantage to either race.
This trick would probably work best with a 2-player map, because with a 3-or-4 player map, there would be a weird effect on Zerg's expansion in midgame. The lack of gas in other mains would hurt Zerg in ZvT, while the extra gas in other naturals would help Zerg in ZvP.
3. Natural gas placed to be easily coverable by spore colonies that cover the main's gas and minerals. In a ZvZ, it's not terribly important to mine natural minerals, so this helps 12-pool against 9-pool by making it easier to defend against the faster Spire.
The natural minerals must still be difficult to cover with static defense in the main, or else Zerg would be hurt in ZvT.
Thoughts on these or other ideas?