This is an algo inspired by "Risk Premia and the VIX Term Structure" by Travis L. Johnson, http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2548050 .
The backtest looked way better before I got rid of all the lookahead bias I accidentally introducted with the use of fetch_csv. This slope idea has some merit though, I think - perhaps it needs to be combined with the "variance risk premium" signal between implied volatility and realized volatility. I have also wondered whether these sorts of strategies' performance varies with the exact theoretical composition of the futures months of the VXX/XIV ETPs. Perhaps they decay/grow fastest when they hold mostly one VIX futures term, or perhaps when they hold a perfect balance?
Anyway, hope this provides some food for thought! I'll be continuing to play with this one, but I might have to switch to using intraday VIX/VXV/VXST/VXMT data (outside of Quantopian).