This sounds like a great idea that could bring a lot of traders to the DEX and adds a lot of value to it as a trading platform, I like the idea of being to lock up any asset and think it will improve liquidity.
If I think BTS is cheap at the minute but don't necessarily want to sell my remaining BTC to buy more, locking it up to borrow might be a sensible middle route.
Similarly guaranteed interest returns should encourage more people to be making bitUSD/bitCNY, especially if there is demand and interest rates are attractive. I think it would bring more fiat in as well - bank rates are terrible at the minute, bitUSD with interest paid via a lending facility has to be a better option.
My only concern would be if we get tangled in something that complicates the DEX's status as they try to say we are acting as a money lender and are therefore subject to X, y and z laws.