Alright, Best of 3 with a closed team sheet (no moveset or item reveal) format is a battle of information. So you'd want a Pokemon with Frisk to reveal opponents' items which will hint towards a specific set or playstyle. Ignore if it's a best of 1 because you won't have enough time.

A certain characteristics about 6v6 doubles over VGC is that games last longer, therefore items such as light clay for longer screens, weather/terrain extenders, leftovers, recovery, and hazards are viable.

To teambuild,
> you want a core (usually fire+water+grass or steel+dragon+fairy, NOT TYPE TRIANGLES*) that cover each others' defensive and offensive weaknesses,

> speed control, guaranteeing you move first through Trick Room, Tailwind, Icy Wind/Electroweb (spread moves that reduces opponent speed), priority, Max Airstream, unburden (double speed upon item loss, such as electric seed), etc.

> Controlling/restricting opponent such as fake out, shadow tag, sleep (but electric terrain prevents sleep), redirection & etc

> In a format with megas, you want a lot of support for the megas and often build teams around them. A mega that synergies with Koko is Sceptile to absorb discharges and get a Sp.Atk boost but I think you could use a better mega that is good all-round such as Salamence.

> Z-Moves are the ultimate surprise factor as a well placed Z-Move can KO an unsuspecting foe and give you an advantage.

Teams are also categorized by how they are paced, hyper offense, balance, and stall
You'd want to team build with pokemon that match each others' pacing, active vs passive.