So here's my first question:
Are you playing Unbound? Because of the way the rules are set up for Battleforged army, and because I *assume* you want your Dominus to be the warlord, you would need one more squad of Servitors.
That's a... tricky question, given the current Mech rules. I am playing a Skitarii Maniple army, however the Tech-Priest Dominus can be fielded as part of a Skitarii army using the new formation from the Start Collecting box. But he is technically, at the moment at least, a Cult Mech unit, which means I should be able to count him as both part of a Skitarii Maniple (and the Warlord of it) and Cult Mech Battle Congregation. So I should, in theory (probably not though) be able to field all these units as one force.
Otherwise... Hmm. So you want something in close combat? What about Space Wolf Void Claws? It's a formation of 5-10 terminators, all with lightning claws. You can add cyclone missiles if you wanted (1 per 5), and they deep strike in turn 1 guaranteed. Oh, and they're WS5 in that formation.
Well I want something that will best complement my force. If that's some melee to address the lack of it in the army then perhaps, but I won't discount more range. Perhaps a heavier more durable range unit would serve me better than additional melee...maybe. Space Wolves is an interesting idea, could go that route.
Ravenguard actually get a better (in my opinion) formation called the Bladewing Brotherhood.
It's 1-2 squads of Vanguard Vets, and 2-4 squads of Assault Marines, and then a captain or chaplain.
The VanVets could be tooled up in all sorts of fun ways: Power Weapons, Lightning Claws, PowerFist/Lightning Claw, Gunslinger (two different types of pistols), etc. The Assault Marines can take flamers, and then in each unit you can give them an Eviscerator or two (depending on skize), which is like a powerfist with free armorbane. And the Captain/Chaplain can take a RG-specific awesome jump pack and some sick lightning claws.
Hm, like the sound of this. Definitely a possibility. So three five-man squads, one VanVets and the other RegAssaults (which I would just use more VanVet figures for because one VanVet box costs the same as a box of RegAssaults, GW's poor sense there benefits us nicely)
What I myself wonder about are GK Interceptors and Purgators as support. Not only would they add to the army, Interceptors would add to it's low melee and Purgators would provide a nice heavy range hammer, but they would also give me some psyker power. Which at the moment the Mechanicus as a faction utterly lacks, both actual psyker ability and anti-psyker ability. A Culexes Assassin would address the latter nicely, but the former is another matter.