Please consider adding this to PBE just to try it out.
Thresh and Mordekaiser are very similar in terms of their nature. They both revolve around death and spirit.
I think that Thresh's Dark Passage should allow an escape from Morde's Realm of Death. A sacrifice of some sort. Thresh collects souls with each death. Maybe a trade, where Thresh would sacrifice 30-40 souls in order to grant the person inside of Realm of Death a way out. Or where Thresh would trade places with the person inside refreshing the cooldown on the Ultimate. Ultimately, I just want more interactions within the game. I know that both champions tend to bug out sometimes, but that should not be that big of a problem since it only would affect them.
Please feel free to comment any suggestions or criticisms of this.
I have mained both champions over the years, and I have at least a million mastery on each, and they seem perfect for such interaction.