Before the show materialized as Teen Titans, it was originally proposed as an adaptation of Young Justice, the successor to the Teen Titans in the comics (which was made because the then-recent Teen Titans (1996) comic was cancelled due to low sales). Ultimately, it didn't pan out, one of the reasons being that much of the complete roster for Young Justice consists of Legacy Characters and relied so heavily on pre-established comic knowledge note the roster has included Robin III, Superboy II, Impulse/Kid Flash II, Wonder Girl II, Arrowette II, Lil Lobo and they feared this would likely be too confusing for non-comic readers, especially the kids the show was ostensibly meant for, and having to manage all these legacies would probably also be difficult for the writers as they'd have make the universe much broader than intended. Hence, the Teen Titans we know today was born. While Young Justice would eventually get its own adaptation that was well-received, it takes some creative liberties with the source material. One can only wonder what Young Justice would've been if the original pitch made it through.
Slade was supposed to team up with the Brotherhood of Evil in season 5 and become a (successful) Starscream. Instead, he gets relegated to a cameo in the final episode due to the season order being cut down from 20 episodes to the standard 13 episodes.
Terra was originally planned to appear in more episodes and the theme song in order to help better build up the idea of her being part of the team so the betrayal would sting more. But due to a time crunch, the plans fell by the wayside and were unable to be used as intended.
Cyborg was originally planned to be a gadgeteer who upgraded himself continually, though the writers became concerned about the idea of that making him too powerful and decided not to incorporate that idea.
There was meant to be a New Teen Titans adaptation in the early 80s but it never got past concept. It was to include Wonder Girl, Raven, Starfire, Cyborg, Changeling and Kid Flash. Wonder Girl was the leader as Robin was busy and in Robin's place would be an original character named the Protector. That said, the closest to such an adaptation would be an animated 1984 anti-drug PSA and an accompanying tie-in comic book.
The DC Animated Universe back in 1996 had conceptualized a Teen Titans show set in the shared continuity, with the starting lineup being the first five members of the comics (Robin, Kid Flash, Aqualad, Wonder Girl and Speedy). This never made it past the concept stage and a brief mention by Batman in Static Shock, and ultimately the show we know today that's not part of the DCAU was made years later instead. Had the DCAU Teen Titans been made, history could've been very different for the franchise.
In the "Mad Mod" episode, a completely different song was meant for the chase scene, sung by none other than Hynden Walch herself. It was replaced with Puffy AmiYumi's song called "K2G". The original song can be found in the extras of the season one DVD release.
As production was wrapping on the series, Sam Register expressed interest in a continuation, in the form of a Retool or a Sequel Series akin to Justice League Unlimited. A proposal document by lead writer Amy Wolfram, producer Glen Murakami, and Cartoon Network executive Sam Register was sent to the network, but ultimately rejected.
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