This years trade deadline is hard to predict at this point because the draft and free agency haven't taken place. It'd be difficult to guess what each teams starting 5 will look like next year, let alone who might be on the trade block. Some teams might struggle and decide to rebuild, some might exceed expectations and look to add a piece or two. There are too many variables to try and predict who might be available with any certainty.
As far as cap space in the summer of 2020, that's still 3 drafts, 2 trade deadlines, and 2 full offseasons away. Way too much can happen in that time. Sure, some contracts will expire that offseason but whether teams sign players to extensions or not before then is too hard to predict. Here's a link to a site that lists contract information. You can see what deals expire then but I wouldn't count on anything being for certain that far out. I'm sure some deals signed this summer will expire then, so they wouldn't be listed yet. Also some younger players will prove themselves in that time and while they may not look great not they could be big time targets that summer. There are way too many variables to try and guess whether the Suns will have cap space then or not either.
http://www.spotrac.com/nba/free-agents/2020/