When the patent plans are broken down, it appears as though Polaris may have a legitimately scary competitor for other players in this recreational niche, including the Campagnia T-Rex, Can-Am Spyder (which is admittedly not a side-by-side) and the Morgan 3 Wheeler.
The Slingshot plans call out a longitudinally oriented, 2.4-liter General Motors-sourced powerplant with a manual transmision, though we're told both hybrid and fully electric power could work as well. The single rear wheel will be the driven one, powered via a belt drive rather than a shaft. Steering is by way of a wheel, rather than handlebars.
The Kneeslider doesn't have any information regarding the timing, or even absolute production intent, for the Slingshot, but seems to believe that Polaris is more likely than not to carry the project forward from here. Our fingers are crossed.