I believe there are some aspects of union that league could consider:
1. Short-arm penalties. This is for trivial infringements (as most league penalties are) and doesn't include a kick into touch; just a tap-and-go. League penalties change the outcomes of too many matches.
2. Try adjudication. When the union ref is in doubt he just asks "Try or no try?" He doesn't have to give an opinion at that stage - only if the video ref doesn't have a definitive answer. If a league ref calls for the video it's obvious he is in doubt, so why does he have to make an initial decision?
3. Knock ons. I get frustrated in every RL game whenever a player drops the ball it is invariable called a knock on, when many are knock backs and play on. Union refs are more 'forgiving'.
4. Scrums. Leagues' are a disgrace. Rugby's are a technical borefest. However, a sensible mix of the two should be adopted. Imagine, a league hooker who actually does something resembling the name of his position?
5. Line outs. These to me are a great sight and one of the more interesting aspects of union. Why not league? I'd love to see Kapow lifting Penny into the stratasphere to catch a throw from Api!