I think most will agree that because of US EPA requirements we probably won't see the same engine options here as those currently available in the global T6 Ranger.
The base engine will probably be a large displacement naturally aspirated 4 cylinder. Yawn.
I keep reading that a lot of people think the 2.3L EcoBoost will be the top of the line gas engine. I suppose that is possible especially considering that it should be competitive with the V6 non-turbo engines offered by the competition.
My guess is that while the 2.3L EB might be competitive with the naturally aspirated V6s it would be a gas hog in any moderately heavy towing or hauling situations and because I do some occasional towing my personal preference would be the 2.7L V6 EcoBoost.
While I don't think Ford has announced the power ratings for the 2nd gen 2.7 in the 2018 F-150, the current 2.7 puts out 325 hp and 375 lb ft. Even at those numbers that engine would far outclass anything offered by the competition which means Ford probably won't offer it. The 2nd gen 2.7 will be upgraded for 2018 to include both port and direct injection which should solve some of the carbon build-up issues on the backside of the intake valves as well as improve overall efficiency. The 2.7 engine block is also a very compact design but the twin turbos may present some packaging issues in the Ranger. The fact that Ford currently offers it in the the Edge and Fusion gives me some hope that it can be made to fit in the T6 Ranger. I assume the engine is transversely mounted in the Edge and Fusion so that is obviously not a direct comparison. However, both of those vehicles should be lighter than a 4x4 crewcab Ranger it's not like Ford would be creating some unprecedented, overpowered monster. The other issue might be production capacity. In other words, could Ford build enough 2.7s to meet demand?
As far as diesel engines go my guess is that Ford will use the same 3.2L 5 cylinder inline Power Stroke turbo diesel that is currently available in the US version of the Transit full-size van. A version of that engine is also currently used in the global T6 Ranger which means it already fits. They could also offer the 3.0L Lion V6 that is slated for the 2018 F-150 but unless that is somehow a much better choice why re-invent the wheel? I assume the V6 turbo diesel engine would have similar packaging issues as a V6 gas turbo.
My opinion is that an EcoBoost engine would be a better choice for my intended use than a diesel. I don't see any advantage with a diesel considering the higher initial cost, higher maintenance costs and higher fuel prices. Unless you are driving more than 25k miles per year and working the truck really hard a diesel makes little sense IMO. If that is what you need then you should probably be looking at a Super Duty F-series.