Daniel Solis wrote an article on an alternate carcassone ruleset. I just finished playing it, so here's what I thought:
It's a bit of a non-starter. The "rise" portion of the game has no intrinsic motivation. Although placing the tiles is fun by itself, the knoledge that the game has a second phase doesn't add anything.
Once you start the "ruins" phase, there's a fast a furious point-scoring frenzy, as completed features are cracked open. The first mover advantage is immense. After a few turns, the game devolves into picking whatever tile has both castle and road to acquire the bonus points. The bonus points could be an interesting tie-breaker, but the amount of points they provide is very low.
All of the completely enclosed features have a strange halo around them, where nobody wants to put their opponents into range to claim the structure. It's a bit like when you play connect 4 against an AI, and you know you've lost, since there's either a even or odd number of irrelevant moves you can make before the game ends. There's no strategy to it.
Additionally, the rules wrinkles surrounding enclosed fields make them almost completely irrelevant, unless you go out of your way to ensure a field is contained. I would simply not score fields, if I were to play again.
There's also the minor problem that you end up sorting your carcassone tiles, since you have to count your roads and castles tiles. Carcassone normally doesn't have to be thouroughly shuffled.
However, I think the first few turns of the ruin phase are fun, so lets run with that and come up with:
Play a totally normal game of carcassone.
Once you've completely removed all your meeples from the board (normally during scoring), phase 2 begins.
Whomever had fewest points at the end up the regular game plays first. Play proceedes clockwise.
On your turn, choose a tile. That tile must come from a castle that is currently complete. That tile is not required to be on the edge of the game. You score points equal to the value all completed castles connected to the chosen tile. Pick up the tile.
Once there are no more completed castles, the invasion is over.