A cameo from Sora's Mom would be cool but it's not really needed.
My worry is like in other Shonen Animes, they'll make Sora's parents really important.
Sora's Mom could end up being Ava for example, it's a cool thing to see in KH fanfics but I don't think it'll be executed properly.
Honestly, I think would add more then it would take away and they don't even have to be important. They could just be normal somebodies honestly and it would be fine. But even if they would end up being important in some way, who cares? Its all in the quality of the writing. Just because something is cliche in its genre, doesn't mean it can't work if you do it well... though, to your point, the series does have a trend of taking the cliches parts of shonen anime and making them more cliche. This especially the case when it comes to the mixed bag of how they write women in the series and exclusion of parents without an explanation (a lot of people don't care and I get it but for a story trying to be as grand and expansive with characters and lore as this, its a bit of a flaw for me personally). But again, that doesn't mean it can't work or can't be improved. The relationship between Sora/Riku which, don't get me wrong, they're the best part of the series but obviously very cliche but since the writing works and Noruma puts effort into them, they are most defined relationship. They way the series treats women is a giant mixed bag but Aqua is probably the best character in the entire series (at least for me anyway, I think she's great). And they are... um... attempting do things with Kairi, so that's something lol
Besides all that, KH's lore is very unique and has the attemptly smart angle of being interesting by also tieing into the emotional core of the story and its characters. Its character arcs and relationships wouldn't be the same without its lore.
At this point in the story, I'm just kind of tired of the whole "friends are my power" angle that the series wants to drive home, or at least, in the way its being told right now. I'm obviously not against it and its one of the series defining themes (hell, its the core of most Shonen series, which narrativelty, is what the series is going for) but personally, I just think the series needs to delve into more dymanic relationships if the story, which is mostly carried by its characters and character relationships more so then its plot (in most cases anyway), wants to evolve past this point.
Although I am also against characters in the past turning out to be THIS CHARACTER OHHHH!! It worked with Xigbar/Luxu but I don't think it would work for any other character. The only way it would work again is if Ava is Kairi's grandmother and even then, that's a stretch and a half but at the very least, it wouldn't be out of nowhere considering the way KH X begins its story and considering that this next saga will, hopefully, put Kairi in the spotlight in the Foreteller Saga in the same way Riku was for the Dark Seeker Saga. But no, Ava being Sora's mom would open up a whole can of worms. Or, God forbid, Sora ends up being the Master of Masters which, by this point in Union Cross, wouldn't make any sense.
Honestly, its not even about the parent angle at this point. I can live without that although I think it's something the series needs but I just think the series needs to redefine its themes as a whole and tell it in a different way. They had something going with Sora in KH3 but its really messy, not well established, and too muddled by its plot to convey whatever theme KH3 was going for. Espeically when it comes to Sora/Kairi and any other character relationship that move past the point of friendship. The series cycling around this friendship angle in the way they are telling is going to get boring, stale, and cliche... if it hasn't already for a lot of people. And if they don't do anything interesting with it, by the end of the next saga, if the characters are in the exact same place as they started, that'll be pretty disappointing for a story that I think could so much better then it is right now.
Like, I just recently watched HunterxHunter and though I don't think KH could reach the storytelling heights of Togashi's work, with Disney at the helm I don't know if it can or Disney will ever allow it too, it has infinite potential. And ending theme of the series in HXH by the final episode is basically "Your friends are your power" but in a much deeper way that resonates a lot better.
Maybe I'm asking too much for a series like Kingdom Hearts to be more then Noruma and Disney thinks it should be. If Yozora and his whole world end up becoming a separate franchise (which I don't know how to feel about that at the moment) where Noruma will REALLY start to flex his creative muscles... regardless, THIS franchise, the one we have right now, has the potential to be something unique and amazing. It already is unique but it just needs... more, ya know?
And better writing, obviously.