I have recently been watching Preach Gaming play through Heart of Thorns for the first time and my oh my has it been entrancing. The zones are so inspired, and so well executed as to capture the vision of the game at that time. It blows me away to this day. It's one of the best gameplay experiences I've ever had.
But I can no longer experience that. I already have all the masteries for Heart of Thorns, so I can easily explore and reach all the secrets, which trivialises it all, and means I don't have to grind the events to get to new places. Yes, I could make a new account, but that would mean leaving behind my characters, my account upgrades, and potentially having to replay the core story, season 1 and season 2, in order for it to feel "right" in my head.
What I would propose is a feature to "reset" your masteries for a particular expansion. This way you can get the authentic day1 experience of playing through an expansion, while keeping your character build and gameplay intact. It would just feel like another endgame activity.
I think that doing this would permanently reset your masteries. So you have to grind them out again.
Why would someone do this? Bearing in mind this could potentially lead to people giving up their mounts and not realising how profoundly they relied on them: simple, you would also get compensated for this. Similarly to how 'prestige'-ing in Call of Duty allowed you to unlock new ranking symbols and callsigns, GW2 could provide additional titles and cosmetics. Or even currency to unlock existing rewards, like working towards a permanent wardrobe unlock, or remote bank access etc.
Additionally, more rewards would be offered for giving up more masteries. I.e. if you go through heart of thorns without mounts, that should provide more rewards since it will take longer and be more challenging!
I'm sure this idea has come up before but it's something that really excited me so wanted to share.