Personally, I had a lot of fun in GT5 by taking drivers at their peak and using them for the Endurance races in underpowered cars. I'd coach them up to big leads, give it a break, come back and check how they were doing, rinse and repeat. Had some fun races at Le Mans and the Nurburgring this way using the slow (by comparison) Audi R10 TDi. I knew the drivers had really hit their stride when they were able to win without any interference from me at all.
I would love to see B-Spec in GT6. It would be a grind though. What with all of the PP limits placed on the game, there wouldn't be many opportunities to place them in a hero car and just let them go.
GT6 is different. There really isn't the "need" to grind for cash the way there was in GT5. Every car is available 24/7. I'm at 35 million credits with few cars left to buy. I'm sure there are many who have basically spent all the the money in the game there is to spend. If B-Spec were introduced and there wasn't a requirement to complete it all, barring engrossing changes, I'm not sure how valuable it would be at this point.
I really just want single-player Endurance races with Grip set to Real. Whatever needs to take place so that can happen is fine by me.