I don't feel like rewards should be based purely on speed of experience gain as that boils down to who can spend most time grinding mostly solo. I'd like to propose a seasonal reward and milestone structure that incentivizes both character efficacy, cooperation with other players and a little bit of luck.
I propose the reward tier and milestone goal be based on the completion of Unnamed Runewords. A bounty is placed for Adventurers who can first acquire a collection of a certain number of specific runes. Once those runes are acquired that player can choose the archetype of item that runeword can be applied to and the effect of the runeword within reason.
While character efficacy would be the most important determining factor in the success of the seasonal run it wouldn't necessarily secure victory in this scenario. Trading for necessary missing runes and luck of the drop rate can play a significant role in determining the winner.
Furthermore if we wanted to expand the season we could require the winner to collect any number of runewords in order to secure victory. The expenditure of runes on each milestone piece would allow those who were a few runes short of the milestone an opportunity to secure a different runeword.