Not an alphabetical or national team but inspired by a conversation with a friend an XI who, not by the fault of perfidious selectors, had Test careers that were much shorter than they likely would have been:
Barry Richards: As is well known his career was destroyed by his nation's vile "Home" policy after one Series but I have no doubt he would have been a great, great player.
George Challenor: The greatest West Indian batsman of his era - and plenty of evidence to suggest he'd have a good Test player at his peak. Unfortunately the West Indies only started playing Tests at the very end of his career when he had tailed off badly.
Archie Jackson: I have seem some judges around at the time say (also at the time) that he was better than Bradman. Although a stretch there is enough evidence to suggest he'd have been a top player.
Graeme Pollock: Luckier than Barry Richards in that he had a 5 year run but what he might have been?
"Collie" Smith: Some rated him of a simlar mark to Sobers. Again, a stretch but certainly a very talented batsman and good spin bowler. Killed in a car crash in 1957 - Sobers and pace bowler Tom Dewdney were in the same car but survived.
Mike Procter: last of the really big losses caused by Apartheid. Scratching the surface of his test career when it was stopped. A highly rated batsman (although I was unlucky and never saw him make runs) and a bowler who, even well into his 30s, was capable a devestating spells.
Jack Crawford: After a fine Ashes tour in 1906/07, especially with the ball, and some Test v South Africa Jack Crawford looked set for a long, outstanding Test career. However, whilst captaining Surrey in 1909 he had a dispute withe the Committee about selections for a match against the Australians. A small argument grew into an explosive row. Crawford resigned from the team and moved to Australia.
Ben Barnett: Between 1896 and 1938 most Australian Test sides had included Jack Kelly, Hanson Carter or Bert Oldfield as the keeper. Barnett took over in the 1938 series and might well have had a long tenure but for WW2 - when he ended up in the infamous Changi POW camp.
Norman Mann: A talented left arm spinner whose career was hit at one end by WW2 and the other end by intimely early death.
Neville Knox A very fast bowler (Jack Hobbs considered hin the best fast bowler he ever saw) his career was very short due to what sounds like stress fractures of the ankles.