Legal ways to un-trap.

[DIN0]

In that case, I suppose my first response was appropriate then - the one where I mentioned several expansions like this. Although once again, I have to emphasize that while these can be used for the purposes you intend them to, they can quite easily be used by the agressor to trap others too, so it is not a one-sided counter-balance against trapping players.

[kothmann]

All we are asking the group is for suggestions on other “legal” ways to defend against the aggressive moves!

Hmm...Exactly what is "legal"?  If I play Inns & Cathedrals, can I leave out the Cathedral tiles?  Can I score Cathedral cities with proportional scoring (divide total points equally among all meeples)?  Can I distribute the Das Fest tiles, 2 tile face-down to each player, so that a trap might be a waste of time?  If you can't use reasonable house rules with official expansions, then I think the question is essentially equivalent to "how can the players who don't like aggressive moves learn to play better to prevent those moves".  But this doesn't seem to be the suggestions you want.  I think you want, "what are some modest house rules that we could convince our aggressive player to accept?"  And I agree that this is an important and very useful question!

I think a helpful way to think about this is to try to reduce the maximum point swing that a single play can create ("lower variance" rules).  For example, if you play with Trade goods and player A has 4 Wine while player B has 3 Wine, and there is a city with 2 Wine, then player B closing that city changes the point differential between A and B by 20 points (A+10,B+0 becomes A+0,B+10).  This is a lot!  We don't like it that way, so one variant we play is that each player scores +2 points for each trade good, except the person who has the majority scores +3.  So, in this example, if player B closes the last city, the point differential is only changed by 13 points (A+12,B+6 becomes A+8,B+15).  We cut the swing almost in half!  Wine is still very important, but it hurts a lot less to lose 13 points than to lose 20, based on a single tile.  (By the way, we also like playing this way because then if one player gets the first 5 Wine, the other players still have a strong incentive to keep collecting Wine, because they prevent the first player from earning even more points!)

I also think players are less upset if they don't end up with ZERO.  So, allow roads with Inns and cities with Cathedrals to count 1-per-tile at the end.  Allow a player to swap their Abbot for a regular meeple that they think is stuck in a road or a city.  Use proportional scoring for farms that are worth more than 9 points.  There are so many options!  I don't think any of these things would change the game so much that it would not still easily be recognizable as "Carcassonne".

I hope you are able to keep having fun with your group of 4 and don't give up on your stubbornly aggressive member.

Oh, and also please keep reporting the results of your experiments.  They are interesting...