Best I could think of is refurbished old war Power Armour that is redesigned to run off of Zrbite.
Maybe you could make a case that we could purchase the ability to 'buy' old AW1 Plasma Weapons that still use elerium based ammo, state that we found an old XCOM stockpile of the ammo so we have a limited ability to field a small amount of the weapons, you could take it a step further and make it that you can 'research' a 'Old XCOM Bunker' research that will grant the player a limited amount of elerium based plasma ammo (20 to 60 units per) and that is the limit of ammo for the few plasma weapons we have.
In theory what'd happen is that the player could gain access to, say, two plasma pistols, four plasma rifles and two heavy plasma guns. As well as about 20 to 30 ammo clips for the three gun types, and that would be all the player can use for the entire game, you cannot buy more or manufacture more, as there is no elerium left to make them. As such you could either sell these items for a tidy profit (It belongs in a museum!) , or keep and use them for surface missions only and gain a slight advantage for so long as the ammo lasts.
Either way, I'll have to play the mod further to form more coherent opinions, this is just my early thoughts on your question.
