One problem we've found with this game is the luck of the draw in getting properties next to each other. The other (bigger)problem is the anticlimax at the end. Once the streetcar runs out there's no more high value buildings to place. The last several turns always seem to be spent just filing up the board with low-point buildings when the winner is already obvious.
So we came up with the following ideas a while back:
1. When you draw cards, you state how many cards you are drawing from which stacks (still a max of 2 per stack) and then pick up those stacks and select the cards you want. You must take the number of cards that you stated.
2. When the last neighbourhood is placed, more streetcars become available (say 11 of them). The first player to place may place a single car as a branch off the existing line, all further cars have to be added to one of the ends of the line.
Have tried no 1 and it lead to a closer game. Not sure if it's better or not. Haven't got round to trying no2 yet, but I plan to use sometrains from TTR and give it a go.
Having seen the ideas on here for new buildings and a new neighbourhood, I'm thinking that there could be a whole set of new "Phase 3" buldings that come into play when the last neighbourhood is placed, along with more streetcars, to liven up the endgame.