For powersets I'd go either Kin/Rad or Son/Rad. As for the costume, maybe, as you say, a robotic 'suit' would work, where he's wearing what looks like a normal business suit with robotic hands/feet and a shiny, Shivan Blue helmet with (once L30) a blue glow around it. For the colour of the suit, can imagine it being brown/beige with a cream lapel.
Since I havn't got access to CoH atm I can't quickly make up a picture, but I'm sure it's for the better as it'll still be uniquely yours.
I am the Blaster, I have filled the role of Tank, Controller and Defender
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I am sure a lot of people can do better but here is my attempt at making something shivan like... not the best but I tried to get some of the essence... but yes auras would definitely be an improvement.
http://zouron.otherworlders.org/coh/nososhivan.jpg
Head: Hoods
Shape: Average 1
Face: Skull/none
Hood: Hood
Detail 1: None
Detail 2: None
Upper Body: Tights
Chest: Tights Smooth/none
Shoulders: Ribs/Basic
Chest Detail: Tree
Gloves: Monstrous/Wrapped
Belt: None
Lower Body: Tight
Tights: Tights smooth/Celtic
Boots: Flat/Tights/Celtic
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You might want to take the Stealth power from the Concealment pool at 6. It makes you slightly transparent (a la Shivan goo), so should help.
I've decided to give this a bash as well. A Shivan is very hard to make in the costume creator so I've taken a bit of creative license with it.
His armour is less mechanical and more organic and allows his 'natural glow' to shine through. Again with the skull and spines to play up to the in game creature but I decided to give him some solid boots at least.
Anyway, here's my effort for you. 
@Dante EU - Union Roleplayer and Altisis Victim
The Militia: Union RP Supergroup - www.themilitia.org.uk
Considering of making Steve an ingame presence.
Now the problem is that Shivans are pretty much unlike anything we can create with the costume creator so here's the challenge...
I assume Steve would be using an armoured suit of somekind to protect his wobbliness...so can someone create something which looks Shivan-like enough while having the advantage of having an armoured-suit to go with it.
Shivans have no real secondary powersets (they have those Radiation melee attacks but we can't get those) so what kind of Primary (since he will be a Fender and thus */Rad blast) would you suggest for an armoured shivan besides going Rad/Rad (already have a Rad/Rad fender so want a tad variation).
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