Originally One-One from the upcoming Cartoon Network show Infinity Train (2019) was going to join the group containing Ben 10, Garnet, Raven and KO and not a stone cameo. But the creators found it to be a cheap way to promote one of the network's new shows.
Originally characters from Class of 3000 (2006) and The Secret Saturdays (2008) were also going to have stone cameos. But couldn't be put in due to the running time. Creator Ian Jones-Quartey even apologized on his Twitter account for not adding any characters from those shows because of this.
The current Cartoon Network "CN" logo is used in the title card to form the "C" and "N" in "Crossover Nexus".
The petrified city is "Cartoon Network City", a city where characters of past, present and future Cartoon Network shows live together as well as old nostalgia stuff from Cartoon Network's past is seen there. Also the reviving a fun and colorful city from stone thing pays tribute to one of creator Ian's favorite childhood movies Return to Oz (1985).