Those of us who pay attention to aviation industry news already know that Nav Canada and the rest of the planet plan on implementing space based ADS-B. In January a SpaceX Falcon 9 rocket launched the final 10 Iridium NEXT satellites into low Earth orbit completing the constellation of 66 operational satellites. Each Iridium NEXT satellite carries an Aireon ADS-B payload. (See AOPA article: https://www.aopa.org/news-and-media/all-news/2019/april/pilot/ads-b-going-global) The advantages of a space based system are obvious. It will soon be possible to provide Air Traffic Control services across the entire planet. Trials over the North Atlantic will begin soon.
There are currently more questions than answers on what this development might mean to US aircraft owners. There is no indication that the FAA will abandon the ground based system of ADS-B towers but at some point in the future space based ADS-B will probably be required in order to utilize the airspace in Canada. The cost of participation in space based ADS-B is a transponder that supports diversity. (Diversity provides antennas on both the top and bottom of the aircraft.) The L3 NGT-9000 has supported diversity since its introduction in April 2015 but requires that the second antenna be installed and a diversity access code be purchased. Garmin has come out with two new ADS-B transponders (GTX 335D and GTX 345D) See AOPA article: https://www.aopa.org/news-and-media/all-news/2019/march/26/garmin-adds-diversity-to-popular-ads-b-transponders. This article indicated that earlier versions of Garmin ADS-B transponders cannot be upgraded to support diversity. This could be bad news for those who have recently installed a Garmin ADS-B transponder if they fly to Canada. The Canada ADS-B requirement could be phased in as early as 2023 but there will likely be a lot of push back on the schedule. The FAA is going slow on satellite ADS-B with plans to evaluate it for enhanced surveillance of aircraft flying in the Caribbean region. I think we can be fairly certain that the FAA will at least embrace satellite ADS-B for aircraft surveillance in oceanic regions.
One of the more fascinating developments associated the Aireon space based ADS-B is free Aircraft Locating and Emergency Response Tracking (ALERT). See article: https://aireon.com/services/aireonalert/. If your ADS-B transponder supports diversity then all you have to do is register with Aireon in order to get this free service.
If I was installing a new ADS-B transponder now I would certainly select one that supports diversity. Space based ADS-B appears to be the future for most of the planet and I suspect that over time possibly even in the United States.