The best bet is a blockchain with assets pegged to the major altcoins.

For example, that's possible in a (mostly) decentralized way with BitShares and in a more centralized way (IOUs) with NXT, Ripple and Ethereum.

But these are not the "real" coins, only tokens that track the value of a particular coin. In NXT and Ethereum you must trust the issuer that he really delivers you the "real" coins if you want to exchange them. In Bitshares it's nearly trustless but the values fluctuate a bit and, if I'm right, there is only BTC as such an asset.