I think Thunderlord will be able to change element based on the Subclass equipped, not like how Murmur does it by switching perk nodes.
This is only possible since they are adding the new subclasses.
I got the idea from how Bishop from X-Men DoFP charges his weapon with his power. (Movie was meh except for Quicksilver)
Edit: changed title to better convey message.
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This quite possible since the instances in which we saw these different element thunderlords is in the hands of different classes. The fire version was in the hands of a titan and the void was in the hands of a hunter.