Atomic Cross chain https://bitcointalk.org/index.php?topic=1364951.0 is not easy, but possible. Additionally side chains shows another method for cross chain.

The key is to make sure that at the high level all the current and likely future blockchains can provide the required import function. Ideally into the native asset type, but some coins like Bitcoin and all its forks, of course dont have any intrinsic assets. but with OP_RETURN data or colored coins, even for blockchains without native asset support can support assets.

I think "simple" things like asset id preservation/mapping and support of a meta-assetid is probably the trickiest issue. Once we have a way to have a universal assetid, then it becomes possible for each blockchain to track it and transfer it and trade it.

It is a bit space intensive, but one way is to combine the issuers pubkey with the assetid it was originally created on. This will typically be between 256 bits to 512 bits. One way to make it more convenient is to map that to an rmd160 hash like bitcoin, which then gets us to 160 bits, or 20 bytes per address. That also allows creating bitcoin-style address out of the universal assetid. To avoid confusion with bitcoin addresses, the address type shouldnt be 0 or 5, best to use a unique address type, maybe 0xff which I havent seen used in any coin yet, but I have seen all of them so it might still conflict.

James