I'll start:
Give me value to mentoring... not just AAs. What if running older instances somehow yielded shards (since you need so many of them these days, especially if you have Alts). Perhaps killing X named bosses in dungeons, or completing Y quest timeline yielded 1 shard instead of (or in addition to) the regular rewards? What if completing a heritage timeline also gave a bonus shard (or two)?
THAT would make me run old content. Would be nice to have a change of pace for earning shards too. If the "instead of" option was granted as certain quest line rewards, it could give newer players the option to get a head start on their shards for the day that they reach 80. After all, to obtain a full set of T2 shard armour plus jewellery you need 200+ of them, and that's just starter gear these days.