Looking at Rugby Union as a whole under a system of hybrid rules most NRL league teams would compete on an even basis with the some of the top Union sides.
The Kangaroos as a unit - not the current team but one consisting of the 19 players that have stood down through injury and what not would easily win the World Cup played under hybrid rules. Throw in lineouts, rucks and mauls and contested scrums they would struggle - completely different rules.
However, given the state of Australian Rugby Union at the moment I think many of the players who were overlooked would have made for a stronger team. Talent wise players like Giffin, Dunning, Baxter and Cannon are more suited to Union however someone like Waugh, Smith, Lyons, Larkham or Mortlock would excel in league without a doubt. Much like someone like Paul Kelly or Laurie Daley would excel if they swapped from League to AFL - Kelly was quite a decent halfback in Wagga before taking up AFL.
In the past players such as O'Connor, Thornett, Summons, Mossop, Lewis, Stuart, Gourley, Kennedy, Price, Fairfax etc. have come across from Union and made a good fist of the conversion going on to play for Australia.
Garrick Morgan, Tiann Strauss, Brett Papworth and Anthony Herbert have been notable players who have suffered from switching codes and have returned.
Lesser names such as Jorgensen, Junee, Harris, Andrew Walker, Andrew Leeds etc have all been great club footballers after switching codes.
There are some exceptional footballers playing for quite a few of the different nations who come from League and would have no problem coming back. There are also players who would perform quite well if they did decide to switch codes. Its not so much the combined talent but the individual footballers......