Patti may have been the ketchup on Doug's fries, but the actress who voiced her is so much more. Constance Shuman has continued to work in television, and you can actually see her as Yoga Jones on Netflix's Orange is the New Black. It's one of those things that you might not have realized at the time, but it makes so much sense you'll wonder why you didn't think of it earlier.
Do you remember Arnold's teacher Mr. Simmons? Unbeknownst to many of the kids watching at the time, he's a quietly important figure in children's programming. According to the show's creators, Mr. Simmons is gay. This makes a lot of sense, and puts a lot of the episode "Arnold's Thanksgiving" into a new context, since it's implied there that Mr. Simmon's "friend" Peter is more than a friend. Funnily enough, he was voiced by Dan Butler, who is probably better known for playing the lecherous womanizer Bob "Bulldog" Briscoe on Frasier.
You likely remember Rocko, he of the modern life, as a beige wallaby, but that almost wasn't the case. Originally, he was supposed to be yellow, but this brought Nickelodeon into conflict with a partnered toy company, who already had a figure with the same bright tint. Not wanting to piss off a valuable source of revenue, Nick altered Rocko's pigment to the now to now-familiar gray-ish color. The only real relic we have of the distinctly more Simpsons-y Rocko is the prototype episode.
Of course, after the pilot aired Nickelodeon dropped the toy idea, making the change irrelevant but permanent. Considering how risqué Rocko's Modern Life would end up being, maybe ditching the toys was a wise move.
SpongeBob Squarepants will never die. Of all the Nicktoons to begin airing in the '90s, it's the only one still making new episodes, and the longest running show of the bunch, surviving almost 18 years(!) as of this writing. That's pretty impressive for a show about a talking sponge.
Angry Beavers is not the most beloved Nicktoon, but had its intended final episode aired, it would probably have been more memorable. The episode was entitled "Bye, Bye Beavers" and the plot included Dagwood and Norbert breaking the fourth wall to discuss their cancellation, and the fact their show could be rerun with essentially no cost to the network. We'll never get to see the episode, as it wasn't animated, but the voicework was finished and you can listen to it on YouTube.