How to make Support classes (Bard and Summoner) unique, viable and needed in every group?

Githal

The most basic thing is that Other classes should not be able to do what the support classes do. But what this includes?
Every class (or almost every) should have some self mobility (be it charge, teleport, move speed increase or ect). Also most will most likely have at least 1 cc (stun/silence/slow/sleep/freeze and ect). So if there is DR on cc - in a 8v8 fight, if every dps uses 1 cc - then the Bard/Summoner using cc on the target will be with 1/20 of the time (if not complete immune).

So 1 way to solve this is give supports cc separate DR than non support spells. For example all dps classes share DR of their stun spell. So if Mage freeze stun you, the next stun you get from rogue will be with half duration. BUT Bard stun will share DR with other bards and summoners, so after mage freeze, Bard can stun for full duration, but if another bard or summoner stun the same target the duration is halved (which dont affect dps stun duraion)

Other thing is that in PVE most bosses will be immune to CC, and as a whole utility like increase ally move speed, or grip ally to your position and other like this wont be very useful in PVE. So their buffs/debuffs should be good enough to compensate in pve situations.

Maybe they can be the only classes with cleanse spells as a unique mechanic.

What do you think and do you have other or better ideas?

Halae

I don't think slotting Summoner into the "support class" category is correct. It's meant to be a jack-of-all-trades class that fills holes in your team comp - if you're missing a healer, they can kinda cover for it. If you're missing a tank, they're capable of taking aggro. They can deal damage if you really need that. And they're capable of specializing their playstyle to suit that kind of hole-filling role, by becoming almost as good at is as a dedicated class. That's not support, that's just versatility. Choosing a Brood Warden (Summoner/Tank) to fill one of the tank slots on your team isn't adding a support character to your setup, you've just got a tank with a displaced health bar. The Brood Warden will be capable of all the tank things, such as CC, general damage output, and distracting the enemy.

You're right to put Bard in the support class role, given it's been stated to be structured around supporting others. But you're just kind of... giving it stuff that it has no need for and isn't part of the definition for the archetype. Bard is a buff-oriented archetype, meant to be the midliner that makes sure everyone else becomes more powerful. Not cleansing (which is a Cleric ability), not CCs, nothing like that. Bards are essential as a support because they make everyone else stronger, not because they can do something nobody else can. There's no reason to give them some super special unique ability beyond that, because buffing your teammates is actually already horrendously powerful.