A technology partnership with Toyota would be much great however Toyota is already in bed with Toyota, and like you said Mercedes are pursuing their own agenda with AMG. This leaves Aston Martin very lonely. It would be terrible for them to face the same fate as Rolls Royce and Bentley prior to German ownership. The Arnage was beautiful during its final years but it was a Technosaurus - an outdated car with underpinnings behind times. Nissan or Honda are probably only two choices Aston Martin has. Closer partnership with Jaguar wouldn't be a good idea as it would end with Aston Martin being Astons in disguise.
Honda or Nissan do not have the fonds. Why can't Toyota do that? The LFA proves they have the know-how and Toyota has the fonds. We could end up with reliable Astons powered by Yamaha engines and Toyota handling (GT86/LFA proved).