Characters from Doug are not included in any current Nicktoons merchandise, due to the merchandising rights being with Disney following their purchase of the show and its' studio in 1995.
As mentioned on the main page, all three of the DreamWorks shows have been excluded from Nick's post-2016 lists and retrospectives. They were produced at Nick Animation Studio and Viacom owns half the copyright to them, so Nick rightfully called them official Nicktoons while they were airing. Then The Lawyers stepped in: DreamWorks stopped licensing its characters to Nick in 2016, when the company was acquired by NBCUniversal and ended its partnership with Viacom.
KaBlam! characters never appear on merchandise and are hardly ever shown in NickSplat promotions as Nick has lost the rights to most of the show's one-shot segments and music videos, making it difficult for it to air in reruns or get any form of home media release without any major edits- they most likely don't want to promote too much of a show that they can barely air anymore. Funko had the show listed in a poll of possible Nick shows for them to make Funko Pop figures of in March 2018, suggesting that Nick might be interested in releasing merchandise for the show.
Christmas In Tattertown was made for the network and was meant to be the pilot to a show called Tattertown, but the show was scrapped due to Ralph Bakshi feeling the short had no potential for a continuation. It's mentioned here because had Bakshi gone through with the original plan, it, not Doug, would've been the first Nicktoon. That said, the short was still successful enough to convince the network to keep going.
Adventure Time was proposed to be a Nicktoon, but was turned down, and it later made it to Cartoon Network. Something similar happened to The Proud Family, which ended up on Disney Channel. Even VeggieTales was proposed as a Nicktoon before being released on home video because Nickelodeon wasnt interested in computer animated shows. Other scrapped Nicktoon pilots werent so lucky... these abandoned shows include Constant Payne, The Modifyers, Skeleton Key, The Crowville Chronicles (by Cosgrove Hall, who made another dropped Nicktoon known only as Trash), and The Weasel Patrol (based on an extremely obscure adult comic book).
Notably, the Big Beast Quintet channel ID was supposed to be made into a full Nicktoon, but this did not come to be. The pilot episode to Stewey the Dog Boy was placed into KaBlam! (it and the other anthology shows are a goldmine of Nicktoon candidate also-rans) AFTER the series was rejected.
Ed, Edd n Eddy was initially pitched to Nickelodeon, but creator Danny Antonucci decided to shop it to Cartoon Network instead after Nickelodeon demanded full creative control.