I've been watching a bunch of old Boy Meets World and Girl Meets World episodes. It's amazing how much the old cast got development in the new series in relatively few scenes.
Topanga mentioned how she put Lauren's letter in the time capsule because she was threatened by it. That whole story arc was about Topanga being broken up about Lauren's feelings for Cory, and Cory trying to convince her that Lauren didn't mean anything to him.
Shawn could have had any girl that he wanted, but all he really wanted was what Cory and Topanga had. He thought he might have it with Angela, but when she broke up with him, he never got over it. And when Cory and Topanga started their own family, he realized there was no place for him anymore. It's why he left the city and why he couldn't stand to see Riley.
Eric was always the most complex character on the show, but he resented how nobody took him seriously, especially Cory, because he was weird and goofy. The senator didn't take him seriously, but Eric got the last laugh and took his senate seat. Also, Cory is now one of the few people who has faith in Eric when in the old show he thought Eric was an idiot just like everyone else.
In fact, Riley is a lot more like Eric in a lot of ways than Cory.