How many 6-mana planeswalkers or greater do you know that have any significant impact across the formats? Not that many.
Mordenkainen definitely has a deep connection with the D&D lore, but as it stands it's just too expensive and its abilities don't seem to go anywhere.
There isn't much love for this planeswalker not even in commander, and not even mentioning other formats. All this will surely make the price drop significantly, so expect it to lose 50 ~ 70% of its current value.
It may find itself in a position only as a collectible for true D&D fans.