Ok, since we are playing sim city I took what you started and refined it. First off, we only need so many freeways to alleviate traffic and a majority of that could be accomplished with east/west connectors. The land costs in Scottsdale and Paradise Valley are too high for them to ever sign off on any new freeways there. I traced these roads to follow the path of least destruction utilizing farm land, river banks, and already enormous roads.
NP - Northern Parkway. This freeway has already been built as far west as the 303 and continuing it along roughly Butler would allow it to be extended to Grand Freeway without too much destruction, it would then follow Grand Freeway South to approx. Maryland because it's far enough south to reduce the amount of eminent domain that would be necessary to connect to the 17.
303 - Here I have connected the 303 to Cave Creek road which is already a massive arterial street with plenty of room to grow and handle even more traffic and become an expressway itself. On the south end the 303 would follow along the Salt River until meeting with the 17.
Grand Freeway - I think we all would like to see Grand Avenue be completed as a limited access freeway which would be near-impossible and cost prohibitive but we are playing fairy tale anyway.
51 - I have extended this to Cave Creek Road with a terminus a few miles south of the 303.
Thunderbird Parkway - Thunderbird is a prime candidate for becoming an east/west freeway. It doesn't have to be of gargantuan nature but much of the roadway is already 6 lanes wide and bordered by open space. Between the 51 and 17 this is the best option.
187 - I have connected the 187 to the 202 which is roughly what is already in the works to a lesser extent.
Sun City Freeway - I think it would be relatively simple to connect the Grand Freeway and the 303 following Skunk Creek. This will help ease congestion through Sun City West and Surprise once Vistancia grows to its capacity.