If we had this argument back in 2003, the obvious answers would have been Sailor, Rogers and Tequiri. All looked good in first grade, but none nowhere near emulating their Union careers after returning. And that was with the benefit of a considerable League pedigree to fall back on.
Rogers I thought came back very polished, and was great for the Titans. Sailor struggled with the flab early on, but proved a good asset on the wing. Tequiri has been a good finisher without doing much else, and came back with a Union wingers work ethic. Remember when we lost them though - it was the start of the big panic about losing players, but in reality Union never really got anyone decent after then - SBW aside (although arguably SBW's mental capacity stopped long before he went).
The point is that these were the elite in a backline far better than the current Wobblies.
The Yawnion world cup has not thrown up a genuine superstar since Jonah Lomu emerged 20 years ago. Meaning somewhere like Keebra Park High is a better breeding ground for RL talent than the Yawnion world cup.
NRL players who struggle to get a rep jumper like Bowen, Hornby, Hilder, Merrit and Burt would kill it in Union.