A while back, my friend Daniel and I decided to make new characters and quest together all the way to max; I made a Storm and he made a Balance. What was really interesting was that he didn't like the Balance school that much in the lower levels, so he chose to use a Death mastery amulet and spam Ship of Fools as his aoe. I thought it was funny at first because I also dislike playing on my max Balance, but when we got to level 50, Daniel chose to keep using his Death mastery amulet... and he never stopped wearing it even now that we are max.
We were both skeptical of the usefulness of a hybrid playstyle past level 50, but the thing is that KI releases a LOT of hybrid gear every year. Literally every pack now has gear with stats for 2 or more schools, and several packs have gear that gives both Balance/Death damage. I then thought about making my Balance a Balance/Fire hybrid character, and I realized I already had gear from the Celestia Spellemental Pack that had both of those schools; Daniel and me also decided to make hybrid pets to accommodate our new playstyles. The point is that we realized with all the new high level gear available now, it would be possible to do a hybrid playstyle and maintain good stats overall.
The overall problem, however, is that we don't have a large selection of off-school spells available to train in the Arcanum. KI added rank 7 spells (Scarecrow isn't trainable for some reason?) to the Arcanum professors back in April 2016, but nothing has been added since and that was nearly 5 years ago. Can the selection of spells be expanded, so that we can train rank 8-10 spells and maybe even some of the Shadow-enhanced ones? I really think hybrid playstyles can become popular and/or useful if we had more of a selection of spells to train from, and it would be helpful in diversifying the playstyles each school has.