I've put some thought into a bitcoin sidechain peg. One major challenge: it turns Bitshares into a high-trust environment. Here's why:
1) We create the two-way peg
2) Various users transfer a large amount, for example 25000 BTC, onto the BTS chain
3) Those bitcoins are held in the BTC chain, controlled by Bitshares witnesses in case of "withdrawal request"
4) This creates a huge bounty for witnesses to go rogue and steal the bitcoins.
Any large-enough group of witnesses could claim to be hacked and steal all the sidechained bitcoins. This isn't possible on the network today, I mean, witnesses can't steal your BTS. Using this would be very scary for me, just because I'm trusting that 20 or 30 people won't be tempted to steal 25 million dollars that they collectively control.
Another potential attack: a majority-by-votes BTS holder can vote in new witnesses for himself. Before, it was scary enough, as he could mess with our blockchain somewhat, with double-spends, etc. But with this sidechain, now he can steal all the bitcoins.
Thoughts?