Grim Dawn and Path of Exile are some of my most played games currently and I believe both games are truly excellent and while they represent the same genre, still as different from each other as somehow possible.
Both are made by quite creative and relative small development teams and both were obviously inspired by the Godfather of all ARPG’s, Diablo II.Both games are also getting new content each year or even every couple of months.
A crossover with Blizzard/Activision seems out of the question but a small crossover through an exchange of a few enemies or a boss as example, 1-2 quests in Grim Dawn that mention the island of PoE maybe or an exchange of a few skills in both games could be very fun. Similar how some of the indie platformer games have characters from other indie games.