Nice suggestions on the storage part of this project.
The encryption methods used would be known, and the source code would be available.
Chosing strong walletpassphrases + using strong encryption on the entire wallet (after it's already protected by a walletpassphrase), and keeping the secrets only on the client side, client could of course make multiple backups of his wallet pass phrase and/or private PGP-key and store it in safe locations.
Point is, if it's done correctly, cracking the encryption and the wallet pass phrase, should be practically impossible.
Just imagine having your wallet pass phrase set to 'Santa Claus loves all children in the world!' and then encrypt it with a private PGP-key holding the keyphrase 'I met my wife Alice at Garden Inn'.
To get access to the wallet and transfer the coins, the attacker would need.
1. Access to the PGP-encrypted wallet.dat (trivial, esp. if you have access to a storage server, or it's stored with bittorrent and you can download it)
2. Access to the private PGP key of the wallet owner. (Very hard, local computer needs to be compromised, and if local computer is compromised anyway, it might be game over anyway)
3. Access to the passphrase for the private PGP key.
4. Now that the attacker has decrypted the wallet, and it can now be acessed, apart from sending coins, which would need the wallet pass phrase.
5. Wallet passhrase would need to be cracked.
Brute force of long passhrases are computably unfeasible, you could check with the calculator here:
https://www.grc.com/haystack.htm. I guess a variant where you combine a dictionary attack with bruteforcing (words instead of letters) would be quicker, but instead of 'I met my wife Alice at Garden inn', using something like '#¤&%554ll_\44DFss-@3-6\\' for the passphrase would take 93.83 billion trillion trillion centuries assuming 1000 brute force attempts pr. second.
To give peace to the user, such a calculator could even be included with the program, so the user could check the strength of his passphrases.
Obviously for such a project, a more specific draft would have to be made, and input from the community would be required.
So in many cases it would be actually a larger likelyhood of the users residence to burn down than having those wallets being cracked. The only ones being afraid of storing their wallet online would be those who doesn't understand the technology properly. Use weak protection and distribute the wallet, and do expect to lose the coins. Done right, the danger is in my view so small that it is non existant.
But anyway, perhaps something like this is already worked on, or partly or fully incorporated in existing software.