I may be hypothesizing but I'm willing to bet if we looked at metrics we would find that Fractals of the Mists are some of the most played content in GW2 to this day. They've been in the game since late 2012 and are still going strong. You can usually either find or create a group at all hours of the day. People like Fractals. I think this is because Fractals of the Mists have literally all of the hallmarks of exceptional group content and they do nearly everything right to cover all of the bases of what the community craves.
I believe all of these amazing traits can be seized upon and used to pave the way toward a new future for Raids. ArenaNet has always been a company that strives to pull players together and GW2 was born with the mindset of tearing down traditional pitfalls that prevent virtual communities from really coming together and sharing a bad kitten experience together. Iterate on what works and make it even more amazing. The hardcore GW2 community has long craved a challenge suited to their mechanical skill (based on feedback found in these forums,) and new players have yearned to just experience something epic and fun when they log in. So, is it possible to give both types of players exactly what they want while also pitting them against insurmountable difficulty in the exact same instance (just for fun?) .. Why not?
3-4 Lane 5-10 man group content leading to an epic climax: Raids would essentially end as 25-30 man encounters for the final boss, but would be accessible/able to complete via two 5 man groups from the start. Each lane would have it's own difficulty (Novice, Veteran, Champion, Legendary ect.) with respective rewards for successfully completing the instance. As an example, the novice lane could have waypoints placed throughout so if you die you can res up at the checkpoint and continue forward. Legendary lanes would be no-down, entire lane resets upon full wipe. HP values, trash mob mechanics ect. could be based relative to the difficulty so each lane would usually take the same amount of time. Maybe 10-12 mins max of hardcore play for the Legendary lane.
The Boss: The bosses would essentially have to be a suicide mission for the Legendary and Champion lane groups. 1 Hit KO's, unblockable 80% health sapping floor damage ticks that make healers cry, all that. The bosses would have a particularly deep-seated urge to annihilate these two groups as fast as possible, and both groups would absolutely need to coordinate on the fly/resort to muscle memory to complete mechanics. In contrast, the novice and veteran lane groups would play supporting roles around the final boss fight areas. Most of the mechanics would be easy for these groups to complete and would grant extremely helpful boons to the champ/legendary groups. These boons wouldn't be required to complete the encounters on Legendary difficulty, however, just nearly required. Should the Legendary and Champion groups succumb to failing their assigned mechanics, they would be sent to the beginning of their lane in a fresh instance with a few extra copper in their wallet while the novice/veteran groups close in to finish off whats left of a downscaled raid boss.
I really think that this kind of raid format would go a long way in catering to each niche of the GW2 community while also bringing them together to complete a common goal. Personally I would be sticking to the Novice lane for eternity. Just some random thoughts!