I think it's no secret that by now playing alts anywhere under Eternal can take forever to find a group. Here's my suggestion:
Weekly quests that require you to play any underpopulated roles for 10 games in X difficulty (X being Any but eternal) and the reward can be something akin to doing 10 dungeons in Eternal on your main. So around 2000 gold, loads of Marks of Fellowship and maybe an item that you can exchange for 1 Splendid Gem of your choice.
These weekly quests would come live after 1 week of each season's start so you wouldn't feel obligated to do them on an alt day 1 for the rewards.