an ending where Miriam takes the throne and castle as master

That would be so cool and interesting, and there could be story elements that would cause Miriam to convince herself that's the best path to take.
it would also add more roleplaying freedom since the player is Miriam, wouldn't the player have control to consider another path other than
what's perceived as "good"? Perhaps Bloodstained could blur good and evil, and make it so gaining power and control, can be for the greater good,
and the castle itself could have possibilities and would be a total waste to destroy.
Even if Miriam is perceived as evil by lesser minds for taking that path, symbolically, it could be designed so right and wrong
are really based on perspective, instead of the black and white stereotyping that stories usually go through.

normally, IGA vania endings boil down to the protagonist defeating antagonist and castle crumbles,
or neutral and they just leave,
or maybe someone goes evil, but their outcome is vague,
or the protagonist sacrifices themself, and/or their friend does. (that sucks)
or the protagonist becomes "pure", and the usual goody two shoes apple pie rainbows event occurs.

an alternate ending really would stand out more with option for Miriam to take absolute power and castle for herself

there's very few games in the industry where the protagonist can switch sides and take a more dominant path, which would allow them to have control
and choose how to use that power, ensuring no one else stands above them in the future.

like in Street of Rage 1, any of the protagonists (including Blaze Fielding) can betray their partner, kill them, kill the antagonist as well, and take over the syndicate as the new leader.
(which can be interpreted in several ways.)

in Contra Hard Corps, there's an ending where any character (including Sheena Etranzi) can join the antagonist's side, and has really epic ending art worth a thousand words.
(which can be perceived as underling or alliance path, but those that choose that path, combining forces with the antagonist, actually opens new possibilities)

in The Last Blade 2, Hibiki can take a path of mercy, or lots of killing and bloodshed, which can change her attitude and she can pursue a more violent lifestyle.
(which is equivalent to a cold assassin for hire, but it still is an interesting and thrilling path to choose from than the usual "avenge parent and go home" deal. )

in Shin Megami Tensei IV Apocalypse, the anarchy path leads towards very dark yet most empowering outcome ever created.

well its up to the developers how they want Bloodstained to be perceived, and if they are gonna stick the status quo perception of good and evil, or
gonna delve further into more open minded possibilities and outcomes.

either way, Miriam should not be punished for whatever path is taken. so light or dark, each path should be rewarding. and of course the darker path should
be naturally more rewarding and empowering since that reflects upon the nature of power being extremely enticing.

I feel this subject and adding something like the title has, could make Bloodstained something deeper instead of so one sided.