Although overall composition is easily a topic I could cover all on its own, making a quick analysis about what your team is capable of can be a big benefit.
Recently, I played a game where my team included both Teemo and Shaco. Usually this is a disaster, and in most cases it would be a lost cause. I locked in Poppy and hoped for the best, except in my case "hoping" means "playing to win." Instead of trying in vain to pressure a top capture point where my team had no chance of winning a fight, I encouraged Teemo and Shaco to lay traps in the jungle and focus on backdooring. The enemy team had a good teamfight composition and played a standard game. When the enemy would send someone after my backdooring buddy, I would be waiting in the jungle for him. I would meet my foe shield-first, slamming him into the wall and stunning, and Shaco would blink to me and help me finish him off. By controlling the jungle and playing to my team's unusual strengths, we won decisively.
The first composition is a teamfight comp. This includes a team of four characters well-suited to teamfighting together. They usually have some kind of ranged harassment, but are not as focused on it as a poking comp. The main focus is on winning 4v4 engagements and also on 2v2/3v3 engagements. One champion in a standard comp is usually a strong ganker, and assists the bottom lane, one is generally a tank/initiator, and the others are usually DPS roles suited to working with the ganker and tank. Ranged carries work well, as do many bruisers and some mages.
The second comp is a poking comp. Some poke comps have good teamfighting, but it is rarely as good as a teamfighting comp where the emphasis is on tandem ultimates to lock down the enemy as much as possible. Poke comps have lots and lots of ranged harassment. Lux is a common choice (and a bit underrated, in my opinion), as are many others. Every team needs a good tank/initiator and Maokai or Blitzcrank make good choices to round out the team. Everyone else, except the bottom lane, should have some kind of great ranged poke. Poke teams wear down other teams, eventually engaging after the entire enemy team is low on health. It is nearly impossible to defend a point against a good poking comp. On the other hand, a forced engagement can be devastating to a poking comp, and the team members generally need to be together to present any real threat.
Another comp is focused on skirmishes, especially 1v1 and 2v2 skirmishes in the jungle. They generally are also good at teamfighting, and can be considered an alternate for the teamfight comp. The emphasis is on good dueling and catching people rather than full-on team engages, though. A skirmish comp is generally horrible at poking, since most poking champs are terrible duelists. It can also be difficult to play a skirmish comp against a teamfighting comp at the enemy's point, as the teamfighting comp probably has better poking and has the benefit of a turret.
The last comp is a mobile backdoor comp, and should be considered a composition of last resort. Mobility and skirmish power are the most important elements of such a composition, and pushing power can also help put pressure on turrets and keep the enemy on the defensive. Backdoor comps are somewhat of a gimmick, but it may be your only option if your team locks in Zilean and Nidalee.
Tanks and initiators
The best tanks on Dominion are similar to those on Summoner's Rift. There are some exceptions; many tanks in Summoner's Rift are chosen for excellence in the laning phase or jungling speed which are not a concern in Dominion. Additionally, death is frequent in Dominion and characters that depend on avoiding death (such as Cho'Gath) are less useful.
As mentioned above, Blitzcrank and Maokai are great harassment tanks. Blitz can single out individual foes, which makes for much more manageable fights, while Mao has tons of harass with his saplings. Malphite also does quite well with his slowing disc.
More general teamfighting initiators include popular choices such as Amumu, Jarvan, Singed, Alistar and Rammus. Blitz and Mao also teamfight exceptionally well.
In addition to the above list, Poppy and Lee Sin make very good skirmishing "tanks," though they deal explosive amounts of damage and are generally built as bruisers. Many bruisers, such as Jax, can also be built tanky to serve as initiators in jungle battles.
Zoning and poking
There are a lot of zoning champions in the League
. Any champion with 700 spell range or greater (that's 600 autoattack range) can potentially zone out anyone with less, especially if the zoning ability doesn't also bring your champion to the enemy. Amumu's bandage toss might have great range, but it is not very good as a zoning tool since it pulls him to the enemy, who is likely with many friends.
Lulu, Diana, Jayce and Urgot are all considered very strong in this category and are frequently (Lulu and Jayce almost always) banned in draft. Kassadin and Kha'Zix, who are considered the top two Dominion champions right now, are also fairly good at ranged poking.
Ganking and roaming
Sometimes, your bottom lane has a bad matchup or the human behind the champion isn't quite up to the test. A fast-moving roamer helps cover that weakness, and even if your bottom laner is winning the battle, a gank can mean a free point neut or capture. Additionally, gankers excel at jungle plays, as their combination of speed, damage, and CC can get to a skirmish quickly to tip the numbers in your favor.
Kassadin and Kha'Zix are considered the top characters largely for their mobility and high damage, and either can build tanky and still be very deadly. Many of the above tanks, such as Blitz and Rammus, make phenomenal gankers. Shen is also a good choice for a ganker, and while he isn't the best choice for an initiator he makes up for it with the ability to teleport to teammates. Pantheon, Twisted Fate, and Nocturne also have amazing gank ability with their global ultimates. Evelynn is one of the best roamers in the game, period. Even if a character doesn't have a lot of mobility, any champion with the ability to suddenly arrive in a critical moment and deliver CC or heavy burst damage can be an effective ganker.
Nothing but the damage
Damage is great, and sometimes damage is the most important thing to add to your team. Ranged carries and mages are the perennial choices, though in Dominion, bruisers are an attractive option. When looking for more damage, check to see if your team has other roles covered first. If you're lacking a bit of tank but need damage too, you should grab a bruiser, like Jax or Wukong (or a mage with tanky items).
If you just want ranged damage, the best choice is Ezreal or Jayce. Most other carries are somewhat weak in Dom. Caitlyn and Vayne can be decent, depending on comp (Vayne is a better duelist and Caity is a great poker). Corki is also fairly good due to his poke and escape.
Sometimes you need magic damage. Kayle is a decent choice for ranged magic damage, and is a surprisingly good duelist (if her ult is up) and harasser. Swain, Ahri, Zyra, and Vlad also work well.
The fifth wheel
Bottom lane is another topic altogether. It is a good idea to make sure that you have gankers that deal opposite damage to the type your bottom laner deals. If you have a Yorick in bottom, run gankers like Evelynn that deal magic damage.
Fitting it all together
Your top team comp wants a single tank/initiator. Another champion should assist the frontline as a secondary tank, tanky DPS, and cleanup character. You can run anyone else in the other two spots, typically damage or a mix of damage and CC. Some supports, such as Taric and Sona, can work well in these spots. As a whole, your team should deal a mix of physical and magical damage. The champions most likely to gank should deal damage of an opposite type to your bottom laner.
If you've only got one champion pick and everyone else insta-locks something, a good rule of thumb is to play bottom if it's not obviously taken, then play a strong initiator if that is not already filled. If both of those roles are filled and your team lacks CC, pick something with good CC. Otherwise, pick a damage dealer that supplements your team. If most of the team is squishy, pick something tanky. If the team is mostly physical, build magical.
Most of my roster is full of champions to fill roles for Dominion. I have a champion for almost any situation (my bottom lane AP mage is Zyra, which is not super ideal, and I don't own Ezreal, Jayce, or Corki so I can't fill an AD carry role very well) and can cover a lot of holes in my team. While getting comfortable a small number of champions is important for lower-level players, at higher levels you really have to be flexible in Dom. Good luck and have fun!
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