Depends on the repaint. Sideswipe repainted/retooled as Red Alert, Deep Cover, Tigertracks and Clampdown would definitely work because those G1 toys are all repaints/retools of G1 Sideswipe anyway (it's the same as say repainting MP Starscream into Skywarp and Thundercracker - simple but it works).
I would be greatly disappointed if they repainted it as Sunstreaker or Breakdown or anything that wasn't based on a Sideswipe repaint. That sort of thing might fly for a Deluxe figure, but I wouldn't like to see it for Masterpiece.
Yeah but remember that Masterpiece is based on G1, and G1 Prowl was a Datsun Fairlady Z (280ZX) police car.
Hopefully if MP Sideswipe sells well it might prompt TakaraTOMY to make more Autobot cars and other 'new' characters (i.e. not another Convoy or Seeker) in the future!
In terms of size, I'd like to see them keep it within Masterpiece size. If I want something in scale with Binaltech I'll play with my Binaltechs!