I may have overshot this number, but now it's more of a challenge!!
Nah, it's just a different type of challenge than designing more of a typical cube. With 30 count, the designer needs to find a way to deal with a supply that gives each player 6+ copies if they want them. The card needs to be useful enough to get drafted instead of drafters just letting it wheel and expecting it to be there in the last picks. That restricts the choice more, but people can find interesting ways to work within that constraint.
I like the idea of putting a great glue card in there as the egg. The new
Marionette Apprentice comes to mind. LadyMapi's ideas for Thraben Apprentice and Voldaren Epicure also seem good. It seems like it would be difficult to pick anything costing more than 2.
The most obvious choice seems like any fetch land, especially
Prismatic Vista. I think that would work well, and I have actually considered ordering a pile of proxy vistas. In fact, I think 30 is a perfectly reasonable number for a 180 cube if you want to mostly take color screw off the table. It's just some extra shuffling, and a designer would need to build it knowing that aggro has good mana and every player can easily splash a color or two.
A cheap burn spell could be a candidate in a cube where every deck can at least splash for bolts. The frustration of red drafters everywhere is people splashing to steal those, so just make that the theme. It woud be funny if it was theoretically possible to build a 30 bolt + 10 mountain deck (or maybe 29 + 11 is optimal). I actually think a Lightning Bolt format could work, though. THAT is a resource that players would be willing to compete for, because 10 bolts can be better than three.