https://www.guildwars2.com/en/news/designing-challenging-content/
It will definitely be interesting to see how this works. GW2 purposely lacks the "Holy Trinity" causing everyone to go full dps, relying on evades and group buffs provided by certain classes. It has had unintended affects like cheese stacking, and class discrimination against Necromancer. It seems Raids will not allow this type of behavior. Necromancer is the closest you can come to a "tank" class, its self sufficient, high HP, and has a "second life bar" in the form of its shroud skill. It looks like they will finally have a rightful place in group content by the description given in the link.
Pure speculation, but I also think thief stealthing will have a vital roll that actually has substance rather than mob skipping. They'll be able to remove aggro from players performing certain rolls in the event chain. If there is an escort event, they can stealth it. The Daredevil elite spec will allow them to survive dangerous encounters with limitless evades.
Likewise, Guardians will continue to provide protection, and tank if need be. Guardian HP is half of what Necro HP is.. but a good guardian player can tank just as good if not better. It just takes more skill by timing blocks and managing condi cleanse ICDs. The new signet elite heal is going to be meta
Elementalists will continue to provide devastating AoE DPS, and group heals. Tempest buffs are quite good, and I hope Ele's will realize how useful they will be despite the sour grapes they have over their elite spec. I have a feeling Tempest was designed for raids. It will force Ele players to play more of a guardian roll, rather than the world beaters they are use to being. Maybe raiding will change their tune on their spec.
Mesmers will have a lot more demanded from them. Mastering Continuum Split is going to be a given. They may have to tank, they will provide the group with unblockable attack buffs, and they will continue to boon strip and reflect. Portal skipping won't be an option in raids... although it may still have its place for niche senerios
Warriors roll will stay the same. Provide group strength, and lay down some DPS. Phalanx Strength builds may not be viable in PvE open world HoT, but I'm sure it will still have a welcomed roll for raids. Especially when bosses will enrage if they are not taken down fast enough. With Berserker self fire field, they will be able to blast finish themselves if all else fails.
Rangers will draw aggro, and can tank if the situation is desperate. I have a feeling their roll could be a lot bigger pending on the Druid reveal. Base ranger won't have a lot to bring to the table as it is. Same goes for Engineer outside of the DPS sinister build. Their elite specs will probably be meta for raids as they will be revealed last after raids were announced.
The most important thing is that every class will have a role to fill. There may be a lot of friction from some players though. Most of us hardcore players have more than one class we are fluent with (Guardian, Necro, Mesmer, Ranger for me )... and taking 5 rangers into a raid will probably be a recipe for failure going against GW2's traditional "play how you want" motto. Raid parties will need to be diverse... there probably won't be a necessary meta, but having more than 3 of anything won't be wanted. The "This is the most optimal build for a speed run" thing won't exist IMO because the raid will feature event chains that can't be rushed through, skipped, or cheesed.