There're clearly structures in the North-West portion of The Tower and there's a massive door that alludes to it as well (with a mech standing around that'll no-doubt allow you to trigger the map when it's unlocked).
Care to speculate about what it will be?
Something to do with the factions?
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I'm not sure, so don't kill me if I'm wrong. It might be the iron banner event, and it turns color when it opens up.