Various problems with this may arise, have you even thought it through? Alright, let's try to picture it, I'm a bit high right now and I'll just try to paint it for myself and others with words, as much as I'll miss a lot of things.
Let's imagine this, a random situation (1) you are a mage. I am a mage. I am an above average mage proficient in spell cleaves, I have at least 2100 3v3 experience and I've been at around 2k solo-queue max rate, worse than my 3v3. I hear about this change and since as a mage I have very little point to duo-queue with a melee class, after all that's all I've been doing until this point (MCH, you get a melee anyway, plus a healer) and I have 2000 soloq rate to show for it. -------- Ok, no point to do that, what if I get a healer? I can play with my discipline priest buddy, I guess? Sadly, I know very few resto shamans... Hmm, if I do that I have a 50/50 chance to get either a melee class (back to point 1 above) or get a caster class. Interesting, I hope I get a warlock so we at least beat every other spell cleave! Oh wait... ------- Why not just go with a warlock and play ML/x then? We'll just have a hard time versus certain melee cleaves! Sounds good. I'll try how the other casters work with me later!
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What do we see? A person playing a spell caster would choose to duo-queue with another spell caster. He would beat most melee/caster/healer compositions composed of solo-queing people, he will have better chances against other spell cleaves who are not duo-queing - people going solo in soloq-queue are in all cases getting wrecked, so what happens?
Let's for a second leave the body of that fragile mage and imagine ourselves as a mighty warrior. I am a warrior now. Let's queue a bit of solo-queue, I want to see what today has in store for me. <Queue pop> Yeaha, got an elemental shaman and a paladin, this is my favourite combo! Oh, this ele I've played with before, have barely lost any games with him, I like the guy. Oh ya, they also added this duo-queue thing again... ****, 5 seconds left, guess I'll ask him for some thundercleave soloqueue climb after the game's over. <Gates open> Gladius: Elemental Shaman/Destruction Warlock/Holy Paladin. Well damn.
1 min later: /w Ele buddy: Yo, that game could've went better but I just died in 1 shadowfury, sorry about that. Anyway, **** happens, wanna do some duo-queue? /w from Ele buddy: Ah, sorry, I already told a warlock I would play with him.
Damn. I guess he has good reason for that though, LS/x is quite strong indeed. I kinda don't see a point in playing soloqueue anymore, if I get paired with a MCH I'll have same chances I had before versus MCH since I doubt anyone would still play as a solo player, if I get a melee I guess I might have better chances vs some MCH teams, worse chances against others, MMH vs. MMH would be a zergfest, I guess the ones with a duo-queue partner tho would have better odds... my chances definitely depend a lot on composition I face and if I play MMH or not, plus these Tier 1 weapons just aren't helping my case at all compared to these caster guys.
What do you see here? Melee classes also have to pick the cleave option. Dig further and, rogues for example are more ****ed than before now. Other DAMAGE DEALING classes like protection warriors (ATC), elemental shamans (LSP), mages (MLS) with -strong- setup options get a huge buff on the other hand.
And healers? Healers get paired up with either duo-queing cleaves or random people, things don't really change in any way for them, meaning it's same old boring ****. A discipline priest can't decide for himself what setup he will play, while melee/caster classes have the choice.
Problem example 1: Elemental shaman invites warlock and queues, queue pops. Since class stacking is not allowed, they get either a paladin, a priest or a druid healer. Result - they wreck everyone, they keep queing the same **** making the ladder dead.
Problem example 2: Holy paladin wants to try something new. Can't.
Problem example 3: Melee weapons being tier 1 making spell cleaves vastly more powerful than their melee counterparts.
Problem example 4: Single melee/caster people, err, those who play it as a single person, as solo-queue are at a disadvantage by default.
Here is what I propose to level all the playing grounds:
Solution: A duo-queue group can only participate in games as an MCH setup, while everyone else queing as a single person has a chance to join a spell/melee cleave variation.
Dunno, thoughts? I kinda lost my line of thought at around the middle of this mess, lol.