General note: my concept of this is more akin to Star Wars: The Old Republic's (SWTOR) companion system than GW1's Heroes.
I had an idea for a “Heroes" system for PvE wherein you can "recruit" certain NPCs into Dragon's Watch and add one or two of them to your party at a time.
This seems like the kind of concept they could build a mastery system around for an expansion, with perks and bonuses to you and those heroes when they're in your party. Perhaps a “commander" mastery that makes their AI smarter as you level. A mastery to be able to edit/swap out their skills or weapons, maybe runes, etc. A mastery to improve their armor and/or weapon rating (rare to exotic, etc).
I could see a tactics system similar to that found in Dragon Age, were you can set up commands and prompts for your companions AI to follow when certain conditions arise.
Altogether this Heroes system would be very customizable, and ANet could have a core set of recruitable characters and perhaps release new ones periodically.
I'd err on the side of creating original companions for this though, or using non-Dragon’s Watch members as your "Heroes." Despite liking them a lot, it'd probably neuter their ability to do anything with them storywise (kill them off, leave Dragon's Watch, change form somehow, etc). The recruitable heroes can just be incidental and non-crucial to the plot of GW2, so they're just kind of tagging along with you.
Would totally love them to be voiced and have personalities and such though, maybe dialogue chains or personal quests you can take them on (maybe to get cooler armor skins for them, or better weapons, etc). Could be very interactive and fun.
I think there are a lot of existing characters that could work really well for this. Ones that were relevant at one time in the story, then kind of faded from the plot despite popularity.
Sayeh al' Rajihd
Surviving Destiny’s Edge members
Any of the Sunspears
NPCs from your Order
It’s also an opportunity to include characters from NPC races and “lesser” races like Tengu, Largos, Skritt, Hylek, Quaggan, Kodan, Centaurs, etc. Heck, even some kind of liberated Awakened (bring back Demmi or Belinda!)
There’s also options from the custom personal story like your Charr’s warband, Asura’s crewe, etc. That might be harder to pull off, but I think there are so many NPCs that could be brought back for a much deserved second chance with this kind of feature.