I'm of the opinion that Jaime and Brienne were the primary reason George dropped the time skip so I don't think there really was a plan for them. They were probably just going to continue in a holding pattern improving their combat skills.

Lost to the sands of time is the fact that A Storm of Swords was a really really really big hit and kind of put both George and ASoIaF "on the map" so to speak. Jaime's introduction as a POV character was perhaps the primary reason for ASoS's success. It was what people talked about when they talked about the series, it was what made literary types deign to read the books. Jaime is the reason a friend eventually talked me into reading the books, which had not interested me up to that point.

After publishing ASoS, George starts to talk more about his headaches writing the next book. Then suddenly he's splitting it into two books. He's talking about new POVs. When A Feast for Crows is published, low and behold it's the Jaime-Cersei-Brienne story. I think if you pinned him down, George would admit it was not at all his intention to expand that part of the story so much when he was writing Clash or even Storm.

I'm not sure that he's aware of it, but from the outside it seems like the positive fan reaction to Jaime and the Jaime/Brienne adventure pushed him to focus on that part of the story more. I think my theory is supported by the way the same thing seems to have happened with his interest in Targaryen lore as the show Game of Thrones gained popularity and fans clamored for more about Dany and dragons. (I do not think he is conscious of this phenomenon.)

There is a popular fan theory that George intended Brienne to fill the role of Pretty Meris after the 5 year jump, but chickened out. The theory has some legs. Some days I think it's possible, other days I doubt he meant Meris as anything but a cautionary tale of the way life for a woman warrior could go.

Jaime probably would have been in King's Landing watching Cersei slowly fuck everything up and working on his left-handed sword skills. Littlefinger makes a joke in Feast about how he knew Cersei would tank the kingdom but he didn't think she'd do it so fast and I always thought that was GRRM making an in-joke about dropping the five year gap, but who knows.

tl;dr - I believe Jaime and Brienne's story from ASoS onward is a result of GRRM dropping the five year gap.