He would only be able to serve for plot connection purposes, he wouldn't be useful for combat purposes 1 bit.
Preskip Luffy beat him 2+ years ago. Luffys only got stronger, and the possibility of Shiki, a grown man who was a bit past his prime getting that much of a power boost to beat him is slim. If anything he's either gotten weaker or stayed at the same power level.
Aren't all characters used for plot purposes? The major ones, I mean. A lot of the One Piece history has come into play, Garp, Rayleigh, Chinjao, Roger, Whitebeard, Blackbeard, Shanks, they're all from Rogers Era. They've become very prominent and important in the story since the post-Ennies Lobby arc. Shanks and Roger prior to that.
You do realize that movie was non-canon and a current time Shiki is likely to be on par with Rayleigh, Magellan and Marco, just below the admirals, right?
If I remember correctly, Oda wanted Shiki's story to put in manga, but didn't know where would it fit, so that's why he made a film.
7th Shichibukai will maybe not have whole arc for himself, but will have some role. If Shiki is 7th Shichibukai, than Oda wouldn't had a problem with "fit Shiki's story" in manga.
Having him as the seventh Shichibukai would solve this problem then
Oda has no say over the plots in movies. So that's all non-canon. Everything in the tenth movie.
Don't u think Oda would have some say in the movies? I mean I also know that in movies when the writers want to implement a Devilfruit they first ask Oda and check if he does not have future plans for it already.. Can't the same be said for characters and their powers?
He has SOME say. But it's limited to approvals and disapprovals of whatever the producers and directors come up with.
It's non-canon and irrelevant to the story. There have been canon characters in non-canon content before even Shiki...