While a currency or commodity based robot would be nice. I think Pascal has previously mentioned that it is more difficult (if not impossible) to build a robot on an ETF that doesn't have an underlying basket of stocks. Correct me if I'm wrong on this Pascal.

I'd like to see a robot with a low correlation to the existing robots. If correlation is very high then one might as well just increase position size in existing robots in my oppinion. GDX was a nice complement to IWM in that regard.

Here are some ETF's and their correlation coefficients relative to the existing robot ETF's.
The closer to zero the lower the correlation.

REMX (Market Vectors Rare Earth/Strategic Metals)
0.57 IWM
0.40 GDX
0.48 Average

TAN (Guggenheim Global Solar Energy)
0.43 IWM
0.10 GDX
0.26 Average

XLE (Select Sector SPDR Fund - Energy Select Sector)
0.67 IWM
0.54 GDX
0.60 Average

So based on these numbers I'd like to see a TAN robot.

The correlation info comes from : http://www.etfscreen.com/corrsym.php?s=TAN
You can change the ETF ticker in the url to see the correcation numbers relative to that ETF.