All characters were originally recommended for IDFB 2. The viewers would recommend characters by commenting on a Reddit thread and then upvoting or down voting other recommended characters. In IDFB 2, only the top 25 most upvoted recommended characters were going to appear in the episode, and the first place winner would've joined the eliminated contestants in the TLC. Instead, top 25 appeared in the aforementioned season 4 episode along with 181 other recommended characters who didn't make the top 25.
In the 2-3.fla file, where all of the recommended characters are contained, there is a guide layer named "brands :(" containing 12 recommended characters based on or referencing brands. These characters were not shown in the episode itself, most likely due to copyright issues.
Cherry Pepsi; Forever991 (based on a cherry-flavored soft drink)
Clorox; ljcool (based on the cleaning product)
Flagy; AmericanCamperYT & BrycePearce (based on the USA flag)
While it may not be confirmed for sure, Sword's appearance may be based off of the Buster Sword, the sword used by Cloud Strife in the Final Fantasy series.
Swiftpenguin78's name is misspelled on Perfume's recommender tag.
Discounting David & all his variations (including Dora), Jacknjellify are the first recommended characters that are actually portrayed as humans instead of stick figures.
Chips is a contestant in the object camp The Race of Children, hosted by Niall Burns.
Hailairhous is a contestant from Battle for Palace Objects and was recommended by its creator, just like when AnimationEpic recommended Balloon and Nickel in BFDI 17 and BFDI 18.
Bang Snaps is a contestant from The Struggle for the World and was recommended by its creator, however, he appears to have a slightly different design, as he has arms in TSFTW, but is armless in BFB.
(Smirk) by ThePirhana12 is allegedly based off of the Skype emote.
Frame by DangerDinO is based off the Adobe Animate keyframe element, which is apart of the program's graphical interface.
Kiwi by LOS-Official was recommended to be from the show Last Object Standing.
Spongy's Mother (Spongette) by LifesLikeCoffee's descriptions on Reddit:
Spongy's Mother (Spongette)-- Just Like Spongy but with blush a cute church hat and tiny red lips!!! ~~~~~ Spongette is also a horrible person just like Donut and you can actually understand her!!!
Spongy (Spongette)-- Mother Spongy aka Spongette has blush red cheeks and is extremely mean!! She can't stand pencil!! She is a huge fan of bfdi and hasn't seen Spongy in years!!
TWOWBot by meduza3's name refers to Cary's object camp Ten Words of Wisdom, which uses books as contestants.
Blocky,Eraser, and Pen., Calculator, Cup, DVD, Fairy Floss, Fossil, Freeze Juice, Green Rocky, Muffin, Phone, Pizza, SodaBottle, and TWOWBot had blanked assets in their poses and hand-drawn ones were made as replacements for the blanked assets. This caused changes to some characters, listed in the following:
Calculator, Fairy Floss, Fossil, Muffin, and Phone originally had arms, while TWOWBot originally had legs.
DVD was originally smiling instead of frowning.
Battery's original design had an entirely black body, pink eyes, aqua mouth, and no arms.