A protoss I just played against

gave me the same idea for a funday monday. He didn't research warp gate throughout the whole game. Instead he just chrono-boosted units out of his gateways.

How about this additional constraint to make sure it doesn't just encourage airtoss. You must have less Stargates than Gateways and less Robos than Gateways. That means if you have four gateways, you may have at most three Stargates and three Robos. Or you could go further and say you must have less Robos and Stargates combined than the number of Gateways. So if you have four Gateways, you may only have say, one Robo and two Stargates, or two Stargates and one Robo.

In a way, Protoss has both the strengths of Zerg and Terran mashed up into one race. They have powerful, cost-efficient units, that must be produced out of production buildings, like Terran, and they're capable of producing their units quickly, like Zerg. Constraining Protoss to having primarily a ground army and not being allowed to research Warp Gate forces them to work with a more Terran-like mindset.