Illusions. Illusion magic should be fun, and reward creativity. I have yet to see that happen in 5e, in any game I've played. The rules are very weak on how things interact with illusions, and in multiple games, the best use they're put to is making obstacles to hide behind. Even then, it's all something that has to be negotiated with the DM up front. No other spell school is so poorly supported.A half page in the PHB, with a dozen examples, would be enough to show the spells work in play as designed.