I'll be trying my best not to suggest any expensive cards, but let me see...
Note: Cards marked with a * are the better cards but come from Innistrad, which means they're going to rotate out of Standard in about six months.
I'm not entirely sure about the spells, but I think the deck's okay, and doesn't use any rares you didn't have. Here's my individual card breakdown:
- The deck needs 24 or more lands so it can cast Stolen Identity properly, which is why I added a fourth guildgate
- Four Stolen Identities can clog up your hand in the early game, so I removed one. With three you're still likely to see a copy by the time you're able to pay six mana, anyway
- Doorkeeper is a decent defensive card that comes down really early and can help you mill if you need to; it also loves being cloned as well
- Lyev Skyknight is an awesome evasive creature that works great with Stolen Identity, since it lets you detain stuff continously, and is unique in that it can detain more than just creatures, so you can completely nullify a planeswalker (amazing against control) or weaken some artifacts with it.
- Soulsworn Spirit/Invisible Stalker are prime Stolen Identity encode targets since they'll always get through, and the Spirit detains stuff as you clone it, stopping their strongest creature dead in its tracks for as long as you want to; the Stalker, on the other hand, doesn't hit as hard but comes down earlier and has hexproof, making it nigh-impossible to kill
- Ascended Lawmage is a bit on the expensive side (mana-wise), but it's hexproof, so it's a lot harder to kill and is therefore a greatcypher carrier
- Mizzium Skin is a better card for protecting your creatures than Dispel/Negate, and the added toughness and overload ability can come in handy every now and then
- Hands of Binding is a strong and cheap cypher card that also bogs down your opponent's assault, buying you time until you can get your Stolen Identities online
- A second Azorius Keyrune will prove very useful in getting to six mana faster (since Prophetic Prism didn't accelerate), and can double as an attacker that dodges sweepers, has evasion and can carry cypher cards in a pinch
I hope you find this suggestions useful. I really like your deck... actually, I think I might build something similar and bring it to my local FNM this friday! I love the deck's concept and how it lends itself well to synergistic card choices.
EDIT: I also removed most of the non-creature counters, since you're ery likely to find the current standard chock-full of creatures. If you'd like, you could replace the Cancel/Dissipates for Essence Scatter, even. If you had some money for rares or slightly more expensive cards, you could get your hands on some Azorius Charms and Moorland Haunts (a rare from Innistrad), which are both pretty cheap and could help your deck somewhat.
Here's an example sideboard as well: