I think this would be a cool feature for Bitshares, and it would tie in nicely with the Bitshares DEX and Bitshares' overall "business plan" of offering financial services. I don't think that it would be something that would be too hard to implement too:
Create a new smartcoin asset class called "prisms" and allow them to be created much cheaper than normal smartcoins/UIAs.
Since the price feeds already exist, it would be easy to get the value of each prism without having to publish a price feed for each and every random prism that was created. For instance, the price feed for Bitcoin, Gold, and USD could be used as-is in an infinite amount of prisms as long as the prism's contents were published on the blockchain like:
Prism1 has 90% bitcoin, 5% gold, and 5% USD
Prism2 has 10% Bitcoin, 80% gold, and 10% USD
... etc.
The same price feeds that already exist can be used to calculate each prism's value autonomously because the values and proportions of each asset in the Prisms are known to all nodes.
Then, separate Prisms from the rest of the Smartcoins/UIAs on the DEX under their own tab or page and allow Bitshares shareholders to be the counterparty of any Prism they want. Just like with Shapeshift's prisms, people can go short or long on any Prism that someone has created, thus people can "copy" the portfolio of seasoned investors or popular well performing portfolios or bet that Bitshares will outperform the Prism.
This would be much more decentralized than the way Shapeshift is doing it, as they are the one and only counterparty to all Prisms that are created. The smart contracts they run their Prisms through are a smoke screen so they can label a centralized service as being decentralized. Furthermore, since everything is done autonomously, I think that Bitshares could kill Prism on the fee aspect of things.