1) Regular, enshrined second-tier competitions. NO f**king around - they are played every year and that's that. Even look at Origin equivalents mid-season with stand alone rep weekends, if it is realistically possible.
2) Money. Sure, playing for PNG in the Pacific Cup might not get you as much as playing for Australia in the 4N but you'll still be nicely rewarded.
3) Concessions for Clubs for players from minnow nations. Sign a PNG boy and get 10% off!!!
4) You can change TO the Big 3 ONCE, and AWAY from the Big 3 ONCE.
I know the last one is the most controversial but I think it's realistic. Example: Uate starts by playing for Fiji. He becomes a superstar in the NRL and is good enough to play for Australia, his adopted country. He changes, which is a realistic thing to expect. Eventually, however, he's not making the Australian team and thinks maybe a guaranteed spot in Fiji is better than rolling the dice with Australia. He changes back. That's it, end of story. No matter if he gets a second wind he is with Fiji now.
You see, not every 18-year old kid is good enough to play for Australia. It gives these youngins a chance to play internationalfooty and do their heritage nation proud, whilst assisting with development by increasing their competitiveness.
Then, it allows those moving towards the back of their careers to return to their roots and give something back to their home once again. Think Civinoceva returning to Fiji for the 2013 WC (wouldn't that be awesome?).
Evnetually, yes, you'd want to bring in a "one nation for life" rule but I don't think it's realistic to do that straight off the bat. This would be the missing link that would allow us to step closer to that over the next decade or so. Maybe set an idealistic date of 2021 (World Cup year) for that final change.
I don't think we can get a perfect system from the point where we currently stand. To even suggest so is unrealistic. Personally, I think that the above is the least worst option that we have available to us.