First, good luck on your project. I'm a big .net c# fan...

Here's a couple resources:
http://www.codeproject.com/Articles/1096320/HiddenBitcoin-High-level-Csharp-Bitcoin-wallet-lib

Libraries:
https://github.com/NicolasDorier/NBitcoin.SPVSample
https://github.com/GeorgeKimionis/BitcoinLib

Wallet comparison:
http://doc.satoshilabs.com/trezor-faq/_images/compared_to.png
http://themerkle.com/bitcoin-wallet-comparison-chart-reviews/

Things I'd like to see in a wallet that I think would be interesting to integrate:
Shamir's secret splitting of PK's
Multifactor authentication
Integrated block explorer
Duress access to limited amount
Recovery seed words
Automated encrypted backups
Offline paper wallet generator