While I like the challenge of Normal and Extreme modes, I'm the kind of player who doesn't like to be timed or rushed. That's why I prefer Endless mode, but it isn't really endless. Not only do you stop getting getting upgrades after 150 passengers/day, but there are still plenty of other limits.
For example, You can only have up to four trains per line. You can only have a certain number of lines, and trains are limited in carriage length. Plus, at 150 passengers/day, the map stops growing. It would be great to see what a huge, sprawling city would look like, or how well a single, super stacked line could handle the passenger load.
I'd love a mode that really doesn't stop, or at least one that expands far beyond Endless mode. There shouldn't necessarily be a goal, other than to have fun and be creative. I can't imagine it'd be too hard to implement. I think it'd be a good idea. Anyone else?