I think concluding "Guilty Gear has to be the next fighting game franchise in Smash because it's the next largest and most prominent one not already in the game" is a bit flawed.
Guilty Gear's popularity is primarily an international, console-based phenomenon built off ASW filling the gap left by Capcom in the early-to-mid 2000s and Strive's success after Fighterz put AWS on the map of a lot of casual fans. However, it's never been a fixture of Japan's arcade scene, which primarily focuses on Capcom and SNK's 2-D fighters and Tekken. Anecdotally, every Japanese arcade I went to that offered fighting games put Capcom, SNK, and Tekken front and center, while I only found Guilty Gear twice, both times in a multi-game machine tucked away in an awkward corner.
It's clear that Sakurai has a lot of veneration and (probably) nostalgia for that generation of arcade gaming, which has heavily influenced how Smash has approached fighting representation in the past. That why I think Nakoruru and, to a lesser extent, Akira are so slept on. They're iconics of that era that have already gotten a nod in Smash built off their appearances from the '90s. While not fighting game characters, I would also include Arle and Marco Rossi as slept on for a similar reason, as Puyo Puyo 2 and the various Metal Slugs are omnipresent at Japanese arcades.