The fact is that the Suns aren't going to get a free agent point guard anywhere near as good as Wall this summer, and there are no max-level power forwards available in free agency next summer (unless you count Kevin Durant as a power forward, and there is zero chance of him coming to the Suns), which means that the Suns' cap space will be useless.
Trade for Wall, draft a power forward with the top 5 pick the Suns are going to get, perhaps sign a vet back-up point guard for the mid-level exemption for insurance in case Wall has trouble coming back from injury, and then let the team develop.
In my opinion, that is a better plan than throwing $25 million a season at the likes of Ricky Rubio or counting on Ja Morant to be a legitimate starting NBA point guard at any time during his rookie deal and filling the power forward hole with an early second round pick or some cheap veteran signing that the Suns could just as easily make under the trade-for-Wall scenario.