We have a 5s team that is already separate to soloq wich is perfectly fine if any friends want to get to gather.
I love the idea of dynamic Q but I can also see that it will cause more problems then good.
SoloQ is what makes this game competitive by putting 3,4,5 people into a solo q will take away that competitiveness and self achievement being able to carry your game and climbing ranked is what makes you a skilled player.
There's also a chance of people being bullied in soloq premades.
Just maybe you could increase dynamic q to 3 players max.
Now having more then 3 premades in soloq will make the climb that much harder as you would get a enemy premades that can also coordinate tactics and Ganks.
The only way dynamic Q will work is if it is kept separate from solo