(adding to Kristo's answer because this is a bit too long for a comment)
We regularly play "SuperUltraMega Carcassone" because we have most of the expansions and two copies of the basic game and we are, frankly, silly. It can take a couple of hours to play that way, but it's kind of fun and means we only have to clean up once. Plus there's the challenge of getting everything on the table, which in the later stages further restricts the moves available. Note that we're playing this for fun rather than competitively.
We've tried adding extra meeples but it doesn't really change the game dynamic in a useful way. With the effectively smaller number of meeples during the game farming becomes a less useful strategy and getting a meeple trapped becomes more of a problem. But those are changes rather than major flaws in my experience.
Having the complete double set of extra meeples causes a different problem - people go back to farming, but there's a real tendency to get into meeple wars over desirable bits of farmland. And those can get quite bitter, and lead to people building their whole game around a make-or-break block of farmland. Which makes the game less fun.
My feeling is that you could perhaps add one or two extra meeples if you're playing with two copies of the base set, but not more than that. Given how playable it is without that I would be reluctant to do it. I'm more likely to suggest taking out one of the sets. The game is well balanced enough that we don't see runaway victories, I think largely because once someone gets 50 points ahead of the rest everyone else gangs up on them. It's very hard to stay ahead when that happens.