There's one thing I should mention right off the bat here: Alicesoft absolutely
loves inserting cameos of pre-existing characters.
Not just cameos from the Rance series, either - they'll do cameos from different games, or even entirely different settings. Characters like Rick Addison, Barres Provence, Patton Misnarge and Kabachahn the Lightning show up in Mamatoto, even though that's not even the same continuity as the Rance series. (they went hog wild with this in Mamanyonyo and Widenyo btw)
This is just how Alicesoft does things. Their way of advertizing their other games is to put in cameos. That's also why Alexander shows up in Rance Quest, and why he actually has a dedicated mission there (along with Bernard Ceramite, who's another one of the very few playable male characters in Rance Quest) - he's actually from the Toushin Toshi series. OK, that's still set in the same setting, but it's still a different series. But some of the bonus characters in Sengoku Rance weren't even from the same continuity (which is at least
somewhat excusable in-story due to dimension travel shenanigans, but even so).
Point is; expect to see cameos when you're playing an Alicesoft game. Lots of them. It's just how they roll.
I think the biggest weakness of the Rance games is that they're too fixated on old characters/fan favourites.
I'd call that a double-edged sword rather than a weakness, though.
Having recurring characters can make a series more appealing and easier to get into if you're already familiar with them. And if the writing is good enough, you can make characters from earlier installments show up and still make them feel natural.
This sort of thing is only bad when you
need to understand that character's backstory in order to make sense of that character's role in the current story. It's only then that such a character will feel out of place.
When it comes to major, established characters, I'd say that the Rance games usually handle this sort of thing pretty well. My introduction to this series was Sengoku Rance, and my first exposure to most of the series' older characters was Kichikuou Rance. To me, Maria Tulip didn't feel out of place in Sengoku. And I felt that characters like Rick, Lia, Patton, Hubert Lipton and Satella all worked quite well in Kichikuou. Especially in Kichikuou Rance, I generally couldn't tell which characters were newly introduced and which were actually from earlier games.
That said; the later Rance games do sometimes feature a lot of cameos of minor characters that
do feel out of place. Stuff that makes you ask "why is this character here, and why is s/he explicitly mentioned by name?". Urza's cameo in Sengoku Rance did that for me; even though she's one of the optional requested reinforcements form Zeth, exactly
why she's a big deal in Zeth is never actually explained in Sengoku. Same for Shizuka's cameo in that same game (she's also just a bonus character there, but still).
The way Maria, Kanami and Leila (the optional reinforcement characters from Leazas) are presented in Sengoku did work, because the game provides just enough background info on them. You get that there's a backstory, but you don't actually need to know that backstory beyond what this particular game tells you in order to get a good feel of who they are and why they're showing up here. But like I said; plenty of other cameo characters aren't handled nearly that well.
Like each new game introduces some new characters but they get 5 min of screentime until they're pushed aside for old characters.
Things aren't quite
that bad. Most of the Rance games have a core cast that includes several major new characters.
Rance 3 and Rance Quest do heavily focus on characters from earlier games. But Collapse of Zeth and Sengoku don't (OK, most of Collapse of Zeth's core cast already debuted in Kichikuou Rance, but that's a different continuity). And even in the case of Rance 3, you don't need to have played Rance 1 and 2 in order to get what's happening in 3.