4th Edition, for instance, added new classes left and right. Every new Player's Handbook came with a bunch of brand new classes with their own features and powers. Many of these have made their way into 5th Edition in one way or another. The 4th Edition shaman bears a striking resemblance to the 5th Edition Circle of the Shepherd druid, for instance. But there are a ton of class and build options from 4th Edition that would be great to see in 5th, either as subclasses for existing classes or as brand new options all their own.
One of the most-often cited examples of a great feature from 4th Edition is the warlord. This martial class functioned as a leader, able to replace the cleric or bard on the team just fine. They were non-magical warriors who could heal others simply by their commanding presence. Their powers were focused on tactics, allowing allies to move out of turn and make extra attacks, allowing for an extremely fun and useful archetype. 5th Edition has a couple of ways to replicate this, but none feel quite so good as the warlord.