They need to bring a small truck into the fold, the key is differentiating it enough from the F150 to not cannibalize F150 sales.
Keys IMHO
Size, it has to noticeably smaller. Fuel efficiency, 5 mpg difference if possible is likely the entry point. Margin, they have to find a way to make money at it and have a viable business plan for the truck to stand on its own. Lastly it needs to be the F100, bring back some heritage and keep in best branding scheme in the industry F-Series.