Zardnaar
Legend
The thing that power source/role did that a lost a lot of long-time fans but I think was really brilliant, was it made classes bottom-up design, where in every other edition of D&D, they had been top-down. What I mean by that is, in other editions of D&D, classes are primarily archetypes, and their mechanical design follows from that fictional concept. In 4e, classes are primarily game constructs, and their story concept follows the design.
I think a proper successor to the ideas 4e tried to explore might not even have classes. Classes were kind of vestigial in 4e, and I think freed of the baggage of being a successor to D&D, You could cut them lose and just have players pick a power source and a combat role role, each of which would grant access to a selection of Powers to choose from at the appropriate levels.
You almost start to see this happening in Essentials, where there were a lot of shared powers between different classes of the same power source; although it also introduced secondary power sources, and eventually even started playing around with stuff like the Berserker that shifted from Striker to Defender when raging. God, I loved Essentials.
Are you familiar with Stat Wars Saga Edition?
It was kind of a 3.5/4E hybrid.