Unfortunately the only use ATM is trading to Litecoin or shitecoin, but I don't see these altcoins being particularly useful. Ultimately I'm looking to create infrastructure that will help exchange between Bitcoin and purposeful currencies, be they local currencies or for novel purposes (like Namecoin).
This is my primary concern with MarketCoin. Assuming we had it built today it wouldn't provide that much value to the current system. It also assumes that all alt-coins will be compatible with bitcoin-style RPC api and scripts. Therefore, any alt-coin that wanted to integrate with MarketCoin would have to use some kind of 'plugin' system or be designed specifically for MarketCoin.
So my question is this: suppose you were to create a new Alt Chain, what features should this chain support for best integration with MarketCoin?