Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This is a meta call. Since farming isn't all that necessary for xp or money, I believe the meta in dominion will settle on 2 top 2 bot and 1 roaming. This role can theoretically be filled by anyone, but there are definitely certain champs that can be optimal. I did put a few builds, but of course you can modify them according to your game. I mostly am just trying to give you the idea of a fast, aware, and dangerous 1v1 roamer.
Runes and Masteries
The roamers role is all about MOVEMENT. You need to be quick, responsive, and aware. While the rest are optional, I think these builds require you get the 3/3 in quickness, and 3 movement quints. I'm not going to focus alot in this area, just understand you have to be fast.
Summoner Spells (the death of flash)
Ghost is an obvious choice. In dominion, ghost is without a doubt the best summoner spell. Flash is always great, however unlike Summoner's Rift, there is absolutely no ledges to jump, and it's rare you'll be in a position where jumping a wall can actually separate you completely. Ghost allows the race to top (if you choose), allows you to catch backdoors and possibly perform the backdoor nasty yourself. CV is almost a must as well (for this role). Supports may or may not find their usefulness in dom, but it definitely makes sense for you. You are going to need to find the enemy roamer, or their backdoor attempt, or just scout for your team. It can also allow you to confirm the location of the 5th to give you time to take storm shield.
WTF boots of mobility!?
YES! Yes yes and more yes. This is what is going to separate you from people who are like "lol wtf cait is bad without berserker greaves". But when you can cap a point and be at another in 9 seconds with the maps speed increase spots, they'll think twice. Pretty much every champ that wants to roam is going to need BoM. It also gets you to the top race that much faster.
Why these guys?
I'm so glad I asked :D. While I obviously haven't been able to play everyone in this role, these are 3 of the ones I believe are the most useful. Caitlyn and Teemo have natural wards (which if you didn't notice aren't available at all). A ward when you're not supposed to have one? Amazing. Another reason is these are all really strong 1v1 fighters. Teemo will hope out of a bush, shoot you a few times, and when you go to chase you're hitting mushrooms all over the place (not to mention when someone goes for their storm shield and gets blown up trolololol). A good TF with the stun can win most fights as well. I hardly need to mention the huge impact of TF revealing everyone, and then choosing almost anywhere in the map to drop in.
Playing the Roamer
Here is the fun part! The roamer can be the initial dasher if required, and should probably go with the other 2 top just for the initial cap. After that it's all about map control. You ARE the map control. Hang around in the bushes, lay traps/shrooms in gank areas, and ensure their storm shield is never used. You will be the designated cover as well. If one of your points is getting pushed with more people than should be there, you'll be the one to even the odds. If a quest pops up, you will be putting all your focus into capping that point as fast as possible and getting back into the mid.
Even though mostly noobs play dominion, I've been testing this design for quite a few games and it's very solid. Feel free to try out other good map control champs (gp, panth, etc) and comment/rate my guide. Again, this guide wasn't really focused on the item builds as much as the design and role of the roamer.