Tokyo_Bunny_8990: It was a combination of many influences. Prince of Persia inspired alot of the platforming and puzzles (which has basically been removed in 2018, the *fictional history* from
Onimusha, and the combat from Devil May Cry with a focus on free form combos.
NuffCatnip: It's sad that Capcom is sitting on the Onimusha IP without doing anything with it (regarding a new game).
There's an anime adaption coming out this or next year if I remember correctly, I hope they launch the anime to build hype for a potential new release.
I've seen threads with 'leaked' documents, but who knows if those are genuine (I hope there is some truth to the rumours of a new Onimusha game in development).
Here's one example of a 'leaked' spreadsheet:
https://www.reddit.com/r/Onimusha/comments/jx703w/new_onimusha_in_development_at_capcom_20222023/
The big reason why the Onimusha franchise is having difficulty is probably a combination of kind of jank difficulty and high cost of remaking (due to royalties) and Capcom already making a killing imo.
Onimusha is a difficult game and I do feel one of the reasons why is the tank controls. Its takes getting used to and although the recent remake for PS4 did get rid of it, a new control scheme that completely removes it might go over better. It is also heavily focused on "parry" timing with issen. However, Dark Souls and difficult games are becoming popular (although Ninja Gaiden's resurgence seems a little stunted) so this is a weak point. However, the Onimusha remaster did do really poorly andf were under Capcom's expectations. Low sales for the remastered Onimusha were driving reasons for why remasters for 2 and 3 were cancelled.
The big issue with Onimusha remakes and reboots is the heavy cost to celebrities. Sanosuke is modelled after a pretty famous Japanese actor, 2 is modelled after a really famous Japanese actor who is deceased, and 3 had Jean Reno (which is honestly pretty cool). Although more games are using celeb models, that added cost for an IP with ok sales when Capcom has better options is not a risk worth taking. It apparently is one of the reasons why only Warlords was remastered instead of all 3 in one "trilogy" set.
Finally, Capcom is already doing amazing with their IPs right now that they dont need the risk. Monster Hunter is a banger franchise that Japan's productivity measurably goes down when a new one is released, Devil May Cry 5 is considered the best in the franchise, Resident Evil is alive again and doing amazing, Street Fighter 6 getting a ton of hype. The flagships are flying high and Capcom may not want to risk investing in a franchise that wont go over well and risk not allocating enough resources to core franchises (lets be honest, there is a greater demand for a RE4 remake than a Onimusha 2 and 3 remake).
Capcom is sitting on some notable IPs that they arnt doing much with, notably Megaman and its spin offs (Battle Network, Legends, X), Dead Rising, Dino Crisis, Okami, and even Ghost Trick) but they are one of the few video game developers that are consistently putting out bangers and have come back into being top tier imo (unlike Square where even their flagship Final Fantasy is kind of mediocre).