That is exactly what I am thinking of. A 'visual frontend' for altcoins you have.
Basically it would become a 'wallet manager' but with an interface that just makes it 'feel' like your coins are actually in the 'omni-wallet' shell. The software furthermore could work with auto-updating new wallets. Because the application is developed centrally theres a few people in charge of looking for updates to wallets (or they get notified by coin-makers, which is ofc. even better).

I am however also concerned about security and wonder if there is a way to sandbox all wallets (akin to a mini virtual machine). I just don't like the idea that wallets and the rest of my computer can interact.
Furthermore I *really* dislike the idea that wallets can access each others files (they are often in the same directory for management's sake, to make things worse).

Ideally the sandbox is two-way : The wallets can only see and access their OWN subdirectory. And nothing in your system can access the wallets because they are encrypted if the users so wishes (this could be a large button/option with mini why-to).

An other option in the wallet-shell could be to make backups of wallet.dat that are automatically zipped with SHA256 and perhaps uploaded or stored somewhere.

In the end many alt-coin wallets cause these problems that need to be solved:
1) Management of assets (list overall balance sheets)
2) Secure assets
3) Backup assets