Firstly, thanks for your work! I dunno if this has been asked before, but since I'm not well-versed in anything Japanese, I thought I should ask someone who knows better. So we already know that Katsuki calling Izuku by his name is pretty normal, and sounds more intimate when you contrast it against his previous usage of "Deku". My question is, if Izuku started calling him "Katsuki" (without any honorifics, as uncharacteristic as it sounds), would this sound more or less intimate than "Kacchan"?
That'd entirely depend on the context. Like, there ARE people who call Katsuki "Katsuki" in the canon, you know.
When we say something is "intimate" or "familiar" in Japanese, we're not just talking about romantic intimacy. We're talking about social circles. Inner circles and outer circles. In the above case, it's just common for younger people to use first names for their peers. You get first names and nicknames in elementary school, and maybe in middle school you keep up the first names or you call new acquaintances by their family names, and then in high school, which is supposed to be training for the adult world, you meet even more new people so most everyone you refer to by family names. But if you've known someone by one name since you were four-years-old, you aren't going to suddenly switch to using their family name. That'd be weird and a shocking addition of formality into a much more familiar relationship.
So the change-up you're describing depends entirely on the context. Maybe Katsuki would ASK Izuku to use his full name because he just doesn't wanna go by the nickname anymore. Maybe Izuku switches to "Katsuki" because he's mad at Katsuki and this switch with no warning or discussion is meant to jar Katsuki into realizing something is wrong (I can't imagine Izuku doing this, but that's an option). I once read a doujinshi where Izuku and Katsuki DISCUSS Izuku switching to using "Katsuki" and he doesn't wanna do it because it feels weird, like he doesn't wanna do it unless they're dating, or it means a big new step in their relationship. Sometimes, not always, in the more mutual scenarios like dating, a change in how the two in the relationship refer to each other is discussed before it happens. "Hey I wanna call you this." "Hey can I call you that?" "I'd like it if you call me this." "If you called me that, it'd be okay with me." And sometimes the thing a person wants to be called by their partner is a cutesy nickname.
So if Izuku were to call him "Katsuki," this change wouldn't be happening in a vacuum. There would be some context surrounding it, and that context would determine how this change would come across.