Metal could be used for production and Crystal for technology and development.
All maps would have NumberOfPlanets/1,5 Crystal Asteroids. Planets that get Crystal Asteroids would have either 1, 2 or max 3 of them. Crystal Harvesters would be replaced with Crystal Mining Stations. Building
(and rebuilding) these Mining Stations would be expensive and slow. Each Station would produce 100 Crystal/s.
Crystal Harvester upgrades would be replaced with different upgrades to the Crystal Mining Stations: Automated Crystal Mining Station which would increase the income by 50%, Mobile CMS which adds engines to the asteroid and it's stations so it can be moved and Armored CMS which would double the health and armor. All of these could be researched. They're upgrades to the normal Station, not replacements.
Everything would be built with metal. Some high tech buildings and/or units could use crystal too.
New stuff that would use crystal:Advanced Science Laboratory. All technologies listed in this post would be unlocked from here. All technologies unlocked from Advanced Science Laboratory would use Crystal, Knowledge and time. Starting a research would be like building a Starship. It would consume resources and time. It could also be canceled at any point. Building the Advanced Science Laboratory would cost Metal AND Crystal and be slow and expensive. It could be moved around like other science labs. It would have high health and be heavily armored.
Warp Gate Command Station. Would use 100 Crystal/s.
Node Lines. Node Lines would basically be a new version of Redirector Rally Point that the player could order ships to by holding control and left clicking. The selected ships would then move to another Node with 10 times of their normal movement speed.
Metal Storage Unit would store up to 75000 Metal and use 5000 Energy.
Crystal Storage Unit would store up to 75000 Crystal and use 5000 Energy.
Plasma Shield. The maximum health of this Shield would be the amount of stored crystal. When turned on this shield would consume 1000 Crystal/second. This shield could be turned off and on by pressing K
(like ships).
Fusion Reactor or Dyson Reactor?
(with a small artificial sun inside) would produce 75000 Energy and consume 100 Crystal every second. This could make using Fortresses in high difficulty and/or low AIP games possible. At the moment Matter Converters kill the economy and thus the ability to rebuild fleet and defenses. Fusion Reactors would use Crystal so it wouldn't affect the ability to rebuild stuff. This would also make Starships.. which have quite high Energy costs.. more viable. Sure they're good already but it's not really possible to unlock a lot of them because they use so much Energy.
I'm sure we could come up with more cool technologies that could use crystal.
AI War actually has 4 resources: Metal, Crystal, Energy and Knowledge. This would also add more mechanics to Energy and Knowledge.