Ice can magnify our inner light into brilliant power. Plus as the story goes on the writers can change anything they want. As of right now we use 3 light based classes. Later on we could easily start trying to gain new forms of power because we can't rely on the dying or evil traveler. Or as we explore farther from the traveler towards pluto, we are going to need to use forms of power that don't rely on the traveler. Even the Dreadnought is considered far from the traveler. There are no set laws that we have in destiny yet, so anything can happen, including random new elements like ice