I have only read 10 odd pages of posts so sorry if anyone else has suggested this but does anyone agree with me?
Other than having a multiplayer mode in which you use the existing clubs and national teams, I believe it would be epic to have a division of online which you would play with your own custom team. To make this idea for online play awesome, i feel you should have to unlock the players you can use.
To do this, i feel the best way to do it would be if you unlocked players for this online mode by unlocking them in the single player mode.
By this i mean that it would be great if, to unlock players they had to be signed with your custom club by the end of a season of the World League (or modern equivalent).
To add difficulty in getting your "perfect squad" it would be awesome if the better players became almost impossible to sign into your club (such as the star players) or making it so that only by playing the WL on the hardest setting could you have a chance of signing the highest rated players.
Also to add more variety, because multiple gamers are likely to pick the same players, i think it would be good if your unlocked players had the same stats as they did at the time you unlocked them (Explained in my "My changes for world league" section below ). This would mean that one persons Richie McCaw would have higher/ lower stats than another persons. This not only extends the longevity of the online mode but also makes you want to repeat the world league over and over again (each time giving you something new or giving you a chance to improve your players for online mode).
Of course this creates problems such as:
1)a gamer having bought the game months after others would have trouble winning matches if he played multiplayer immediately
2)what players would your team start with?
3)You wouldn't be able to play as your favorite club/ international team in this mode
But now to answer those:
1)you would still be able to play online using the other path of the online (using existing clubs/teams)
2)you would be given "finnances" with which to buy basic players(division 2 players for example)
3) see answer for 1)
My changes for world league:
-A slightly different game mode through which you could play with your custom team.
-A season format where if you picked a NH club you would play in the local tournament(magners league, top 14, Aviva Premiership, etc) and also in a knockout competition ( Heineken Cup, European Challenge Cup,) and if you picked a SH club you would compete in the Currie cup, ANZ cup and an Aussie equivalent and also competed in the Super 15.
-A system in which player stats would change depending on how a player performs, how good your training setup is, etc. This means that if you overplay a player(every match of the season) he would slowly become more tired so his stats would gradually lower and he'd become more prone to injury. Recovering from injury would make the player have almost basic stats until he recovered etc.
-Making player signings region based (a current All black would be very reluctant to move to Europe and europeans would be reluctant to go south)
-Adding the option to play international/overseas club friendlies (for example: Munster play against the AB, South Africa played Leicester and Saracens, etc.) as an occasion to gain some money or to add stats to your players.
I think that will do for now. Personally, i think it would do rugby fans justice to have a setup like this but im just one person.
Keep up the creative flow