It doesn't really matter how a sequel is titled, as long as the public is happy and coughs up to watch.
The Hitman's Wife's Bodyguard is an awkward title. But as long as you're familiar with its predecessor, The Hitman's Bodyguard, (streaming on Amazon Prime) you could reasonably, and correctly, assume it's a sequel. The title is mildly inventive but the formula could get very cumbersome if there are more sequels: we might end up down the track with something like The Hitman's Wife's Aunt's Cousin's Godson's Bodyguard.
The quick and lazy way to title a sequel - let's avoid the series/sequel debate for now - is to add a Roman or Arabic numeral. Think of movies like The Godfather Part II (streaming on Stan) and Jaws 2 (streaming on Foxtel Go).
The latter is an interesting case. Initially pitched as a spoof, apparently - called Jaws 3, People 0 - the second sequel to Jaws (streaming on Stan) was titled Jaws 3-D (continuing the sequence while acknowledging the technical addition: a "flat" version, Jaws 3, was apparently also released). The fourth entry was Jaws: the Revenge, sans 4. In Back to the Future, one jokey element in the "future" was a movie theatre advertising Jaws 19. Other series might be more likely to reach that point.
One such is The Fast and the Furious franchise - or The Fast Saga, as it is more loftily called. It has had fun with nomenclature. The Fast and the Furious (2001, streaming on Foxtel Go) was followed by the cutely titled 2 Fast 2 Furious, then came a break from numbering with The Fast and the Furious: Tokyo Drift and the lazy Fast & Furious before the numbering resumed with the fifth entry, Fast Five. And there have been more, most recently F9. The filmmakers' inventiveness knows no bounds.
There were monster series, or sequels - often made by different studios like Universal and Hammer - with a name or names or recurring words in the title. In horror we've had The Mummy, The Mummy's Tomb et cetera; Frankenstein, Bride of Frankenstein et al; Dracula, Scars of Dracula and so forth. Sometimes these could be misleading - most horror fans know Frankenstein is the doctor, not the unnamed monster, but this sometimes becomes blurred. And in Brides of Dracula, the bloodsucking Count did not appear, but that film's vampire, Baron Meinster, was hardly a household name.
Some sequels vacillate between Arabic and Roman numerals, with or without secondary titles. The Halloween horror series is an example: Halloween (1978, streaming on Stan) was followed by Halloween II and the subtitled - because it was a departure from its predecessors - Halloween III: Season of the Witch. Killer Michael Myers returned in Halloween 4 and 5, then Halloween: The Curse of Michael Myers, then Halloween H20: Twenty Years Later.
The fourth Karate Kid film, The Next Karate Kid, signalled a change in the series' direction since Daniel-san was no longer the pupil of Mr Miyagi (Pat Morita). The actor who played him, Ralph Macchio, was in his 30s, almost old enough to to be the father of a next-generation Karate Kid. The "next" one was in fact played by future double Oscar winner Hilary Swank.
Sticking with sport, Sylvester Stallone's creation Rocky (1976, streaming on Stan) was followed by Rocky II, III, IV and V, the last in 1990. Rocky Balboa (2006) took a while to come and then the series' focus shifted to the son of Rocky's old rival and friend Apollo Creed. The ageing Rocky took a secondary role to young Adonis in Creed (2015) and Creed II (2018) with another entry scheduled to come.
Another Stallone franchise, dealing with the exploits of John Rambo, began in 1982 with First Blood, which was followed in 1985 by Rambo: First Blood Part II (just in case people forgot), then in 1988 by Rambo III. It took 20 years for Stallone to get around to the plainly titled Rambo (aka John Rambo in some markets), ending - maybe - with Rambo: Last Blood (2019).
Star Wars (streaming on Disney+) was originally titled just that in 1977 but was reissued as Star Wars Episode IV: A New Hope as part of the middle trilogy (which was made first!), putting it in line with the sequels and prequels, during which there were also (technically) prequels that were not part of the main series, Solo and Rogue One, both sometimes subtitled "A Star Wars Story".
The sequels to Dirty Harry (streaming on Netflix) had completely different titles - Magnum Force (1973), The Enforcer (1976), Sudden Impact (1983), The Dead Pool (1988) - but Clint Eastwood as the gun-toting cop Harry Callahan was the constant and what else was needed? Eastwood played two-fisted Philo Beddoe in both Every Which Way But Loose (1978) and its sequel Any Which Way You Can (1980), where apart from his presence the titles sounded vaguely similar.
Sidney Poitier directed himself and Bill Cosby in Uptown Saturday Night (1974) and its follow-ups, Let's Do It Again (1975) and A Piece of the Action (1977). The characters and situations were different but these comedy crime films are considered a trilogy of sorts - maybe in the manner of "an equal, not a sequel" as star and co-writer John Cleese compared Fierce Creatures to the earlier A Fish Called Wanda (the films shared many crew and cast members but had different stories and characters).
Inventive or lazy, it doesn't really matter how a sequel is titled, as long as the public is happy and coughs up to watch.