I hope anet really considers this suggestion, it would make this game 100x better for me.
I think GW2 innovated a lot on PvE and done it well for the most part (getting insta shot in dungeons or Living World events gets old, even though I like the no trinity concept), while GW1 innovated on PvP and had an excellent system from all the freedom of creating builds.
Essentially, I feel gw2 really needs these two improvements:
1) Bring back a proper interrupt system. This was the key to PvP and made it a lot more interesting, as well as PvE bosses.
2) Bring back build freedom
I understand that having complete freedom to make builds is actually bad: it becomes impossible to balance, and we are back to a minimal skill slot system like we had in GW1 (only 8 to 10).
However, if anet adds more skill slots in the future reserved especially for skills that are shared between classes, you could keep the balance and unique feeling of each classes on the current skill slots we have now, and then balance the free slot skills separately.
This would also boost your ideal of “horizontal progression”, since each skill added to these new slots would immediately be shared between more classes, instead of having to work solely on new weapons that only add skills to one class (along with balance issues) and are a LOT more work.
You could even prepare these new skill slots to be part of an interrupt system, showing their casting icon (like in gw1) and letting other players interrupt them. These could be skills that are spammed less, meaning to be used more when the time calls, adding more strategy to the game. Right now PvP is mostly spamming skills, so these differential skill slots could also remedy that.
Finally, it´s just sad seeing Guild Wars being like every MMO out there now, where if you want to have say a pet, you have to stick to only one class and can’t make hybrids anymore. It just makes it less fun, less interesting to develop one character when you are too limited on what he can use. Giving us a bunch of skills shared between classes would make me play my Elementalist with a lot more joy.
Honestly, skill customization is the core of any Stat based game probably to most people, and it is sad that the Guild Wars brand became a bit more generic with it when it was the most complex one with GW1