This post is largely just a terminological update, exploring the subject of what makes it possible for the launch of a spinoff character to qualify as a "proto-crossover." In the 2022 essay STATUTE OF LIMITATIONS ON STATURE, I explained my view as to why the early appearances of certain comics-spinoffs, such as The Black Panther, qualified as proto-crossovers while others, such as Adam Warlock, did not.
The logic set forth in STATUTE remains intact, but I came across the word "through-line" that serves to describe the difference in the two types of spinoffs. The Merriam Webster definition is as follows:
a common or consistent element or theme shared by items in a series or by parts of a whole
The relevant "element" is that of intentionality: whether or not one can show a probable intention of the creator(s) plan to use a character again in either a Prime or Sub role. In the case of the two heroes mentioned, there are numerous textual clues as to editor Stan Lee's plans to use the Panther again in a superordinate role, and those textual elements comprise a 'through-line" linking his early subordinate appearances to his slightly later superordinate status. In contrast, there are no such clues linking Warlock's subordinate appearances to his later starring status, so the former Sub appearances have no through-line and so do not have the status of proto-crossovers.
The same principle applies to the essay example of the Green Goblin. The Goblin is introduced as a new Sub in the cosmos of Spider-Man, while his partners, the Enforcers, are an ensemble-team who collectievly make up an "old" and established Sub. Thus, the initial story possesses a through-line to all of the Goblin's future appearances. However, he's an "old" villain by the time he encounters the "new Sub" Crime Master. But Crime Master will not make future appearances in the Spider-cosmos, so there is no through-line and his appearance alongside the Goblin may be called a villain-mashup but not a villain-crossover.
In STATUTE I used Frasier Crane as an example of a character who was selected to be a spinoff character from CHEERS. Frasier made regular appearances in his Sub status on CHEERS, as opposed to the brief and scattershot appearances of Warlock in two separate Marvel features. Nevertheless, there's no suggestion of a through-line in episodes of CHEERS that Frasier was going to be launched in his own series.
The spinoff of the show ANGEL from that of the BUFFY THE VAMPIRE SLAYER program is arguably a little more complex. The character of Angel is introduced as a mystery-man who comes into Buffy Summers' life in the first episode of her eponymous TV show, and he functions, like Buffy's other confidantes, as part of her bonded ensemble. (In an earlier essay, I argued that Buffy was a Prime and that her confidantes were Subs, but since reviewing all of the BUFFY episodes I've reversed myself on that statement.) So Angel became a Prime in that first episode, as much as characters like Willow, Xander and Giles, and there's no need to see him as any sort of crossover, proto or otherwise, when he branches off into his own program. However, after he gets his own show, any appearance he or one of his ensemble-mates made on BUFFY became a crossover, and vice versa with respect to BUFFY characters on ANGEL.
The BUFFY Sub character Spike is even more involved. He's introduced as a pure Sub in the show's second season and continues in that status. The character's enormous popularity led to his becoming a regular member of the ensemble in the fourth season, though he was in the nature of a "opposed ensemble-character" after the nature of those described here. The transformation of Spike to said status is first set up in the 1999 episode "Wild at Heart." This episode, loosely inducting Spike into the ensemble, is the only one to qualify as a crossover due to a new "through-line" that affects all of Spike's future appearances. But only the first such episode that changes Spike's status gains a crossover-vibe, since only the first "phase shift" foregrounds Spike's acquisition of collective stature, as described in INDIVIDUAL AND COLLECTIVE STATURE.