For me, the solution is simple. Fix the matchmaking. Everyone can feel the satisfaction of having raced 'well' if they are competing on a level playing field.
Matchmaking isn't broken. When there are enough people to go around, the matchmaking works as intended. I think what you may be saying is match people ONLY against people of their own skill level, but that limits your ability to move up the ranks.
As I have been saying since the release of GT League, the ONLY way to make it actually good is to allow online play within the events. Little big planet had it right. You could play alone or you could allow people to play along with you.
GT League races should have differing participant limits, like 8 players for Sunday Cup, etc. Even allow each player to set how many real people can participate and the AI can make up the numbers as needed. Have a setting that allows open access(with proper matchmaking)/only friends/or offline only. Add a check box (like the lobbies) that only shows events with friends so you can join in their races.
I think that's the best of both worlds. What I hate most about the lobbies is that you never really know what you are in for. Hosts leave, they take forever to start races, everyone runs a different setting, some run BoP, some don't, some run RSS tires, some sun RH, some run SS....it's all over the place and a mess. I almost never go into the lobbies now because of the lack of predictability.
I think GT League needs more limits on it, so you can't go and run Tomahawks at max power during a Sunday Cup race or some other such idiocy, but there is TOTALLY a venue for everyone. Let people run max set ups in "Like the Wind". Have BoP limits in the Gr4 and GR3 Cups, etc, etc. GT League can be set up so that there are events to make everyone happy, and adding friends to the mix will only make it better.
Leave Sport Mode as the place to earn your DR/SR rank. Leave GT League as the place to apply your DR/SR rank.