[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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Well with the cold heart giving the visual of freezing any add I think a cyro element would be nice As a side note it should have an special debuff which as long as you keep damaging with the cryo element the speed of whoever your shooting goes down