Funnily enough, Antasma is the only villain genuinly scare Mario and Luigi (he make even Luigi's cry out of fear) while against Bowser they go "Meh".
Speaking of nightmare fuel, since i saw Nightmare's nes sprites (who funnily have a slight ressemblence to Dracula), i was thinking if Antasma came back as that:
Basically current Antasma is: Nightmare+all mario villains
So remove mario villains traits but play up the dracula trait of Nes Nightmare, add Dark Matter's swordsman traits (specifically his hair and maybe the visor) and Darkrai's overal darkness feel.
Personaly, all i want is that they downplay the Cackletta ressemblence, it's good for an anniversary homage but should he come back, i would like to see the "hey it's a male cackletta, Antasma look totally a male Cackletta, i through it was Cackletta" posts gone and Antasma going from "generic Dracula rip-off with Cackletta face and other villains traits" to his own unique character with an unique and memorable design like the other villains.
I'm ok with antasma having nods to other villains but make it more subtle and less obvious, so it can help making the character more unique rather than making him look like a rip-off:
Cackletta: Remove the ressemblence and the similarity, it's way too much, they can give Antasma a tall body structure like her but in combination with Fawful's body, so it look like he has an unique tall body structure. In addition his bat attack should be made of darkness and ressemble more of something like a kobat or a minikyura.
Elder Shroob princess: give Antasma the ability to use his vines like her tentacles, in addition he can drink Vim and "Antasma-ify" his opponents (like how characters are shroobified or fawfulized)
Dark Fawful: Make Antasma use a similar advanced technology and maybe his portal power
Dark Star : Give him the 8 spikes like in his normal design but make it around his head so it look like Dark Matter swordsman's hair, make him able to copy his opponents but based on their worst nightmares rather than their DNA. Maybe give him a black-blue coloration like the dark star core form.
Count Bleck: Give him a facial feature which would ressemble Blumiere (like the one in Mario kun) but much more Dracula-ish (in addition of having Nightmare's traits), give him blue-ish color sheme and some vague mention about living in a dark tribe secluded from the rest of the world.
Bonus addition: Give him the ability to turn his cape into bat wings like meta knight, would fit with his bat motif without ressembling too much to Cacketta and give him a devil vibe, give him a shooting stars attack (all the villains have a shooting star attack), a more impressive antasman's scepter, and Boo-like fangs to match his vampire apparence
They should really look after Kirby, they are pretty good when it come giving traits of other villains to a new one, yet still keeping it unique, see Magolor for example.
This would make a good final boss idea, he forget his true form and what he was supposed to do, turning him into some mad abomination bend on destroying the world.
Yeah, essentialy it signal you are in Antasma's world now.
Also i'm thinking, like Nightmare make him have an ornament but the ornament is actually his mind eye.
As for the angel form i've mentioned earlier which he would take to manipulate the shroobs and others (helping the devil vibe) well, look at this: www.pixiv.net/member_illust.php?mode=manga&illust_id=28356392 Maybe not something exactly like that but at least something angelic and innocent, until he drop the act.
He basically a white angelic looking being with dot eyes, angel-like wings and a big smile, then he open his mouth further turning the smile into his true eyes, the wings become his hair and the white body change into his true body, really like Zero-two's false form.