[spoiler]i feel like Cryo would be most likely just because the current Energies we have are based on the four universal forces. Void being Gravity, Arc Electromagnetism and Solar is Strong Nuclear Force. The only Force of the four not used is Weak Nuclear force which involves the decay of energy (in the most roughest of explanations) so I think Ice would make sense as the absolute decay of energy would lead to absolute zero. Plus it would make so much sense thematically if strong nuclear force is fire, hot and full of energy and weak nuclear force is Cold, icy and devoid of energy. But that's just my opinion. [/spoiler]
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Soil. Definitely soil... we need to be able to soil other players ourselves instead of letting bungie have all the fun