1) The ability to create transactions from an offline wallet. This way I don't have to get the unsigned transaction from my online, watch-only wallet. I can verify that I have sufficient balance myself, and of course you could add a warning and require the user to check a box before this is allowed.
To add some details to what the others have said about the difficulties of doing this, see
https://en.bitcoin.it/wiki/TransactionsIn order to create a transaction offline, you'd need to not just tell the offline wallet that you have sufficient balance, but you'd need to tell it one or more (enough to equal or be greater than your desired output, less transaction fee) transaction outputs to spend. For your typical transaction, this means:
One (or more) 256-bit txid to spend (if it's a from another transaction the offline wallet already knows about, e.g. because it was a change output, you could simply choose it from the list)
One (or more) address from your wallet (could be chosen from the list of addresses in your wallets), being the address to spend corresponding to each txid you're spending
The address(es) you wish to spend to
Transmitting all of this data manually would be tedious, to say the least.