I'll vote for a recreation of the Cock o' The North in its original condition, but with the suggestion that the team modernises the valve gear and pony truck lateral control arrangements. These seem to have been the weaknesses that prevented it from being the great engine it should have been. As to its looks, it has always been my favourite of all the big engines. I'm much too young - honestly! - to have seen it, but my father saw it and admired it on the Aberdeen trains when he lived in Cupar.
It would be a very expensive engine to recreate, so perhaps we should also dream about others with fewer wheels and smaller boilers. Despite my fondness for the Highland Railway, the demand is probably for faster main line locomotives than they had, so good candidates might be the Caledonian's Dunalastair !V, and the North Eastern's lovely Worsdell 4-4-0s. How about the M1, with its 7' 1" drivers, and built especially for racing?
That's a bit of a diversion... I'd like to see a reasonably priced P2 in 4mm scale as well as 12 inches to the foot, but like the full size version, it would be expensive to produce, and restricted in its use.