Team composition is an incredibly important aspect of any competitive team game. Look at any MOBA (LoL, DOTA) or shooter (CoD, Halo, TF2), and you'll see that one of the main reasons certain teams are so successful is because their team comp effectively and efficiently channels the strengths of each player.
In my Splat Roller guide (check it out here:
http://squidboards.com/guides/charge-a-guide-to-the-splat-roller.3/), I talk about how Splat Rollers benefit from having a Splattershot-type weapon to back them up. This is because the Roller lacks so much range, limiting its retreat and approach options.
What I anticipate we'll see in high-level play is a mixture of long-range and short range weapons. As
@thefro pointed out, Splat Chargers are going to be great assets to teams during Splat Zones, as the long range allows players to protect the Splat Zone from enemies. Splat chargers will also be able to act as supports, laying down ink trails for teammates at the start of each game.
Brush users will more than likely act as scouts for the team. With their incredible speed, and access to the Sprinkler sub weapon, (which spreads ink within a wide radius, effectively building up the Brush user's special meter), Brushes can run in and out of enemy fire, while also laying down an initial trail of ink for teammates to move about within.
Finally, Blaster users will want to take advantage of their explosives rounds to expose enemy whereabouts. Using the Gamepad, players can see where enemy ink is being placed. Blasters will want to patrol narrow hallways and choke points, keeping enemies out and pressuring them back to their bases or teammates.
The key to any good team composition is the ability for the team to keep pressure on the opponents. I'm excited to see how the meta will develop, and how teams will adapt during high-level matches.