Kingdom Hearts has a bit of an identity problem, and adding another mobile game is not going to fix that. The upcoming title is set to feature Xehanort. the constant antagonist of the series thus far.
Fans originally expected Kingdom Hearts to be a trilogy of sorts, causing confusion when spinoff after spinoff was released. The fact that a great deal of lore was confined to a mobile title that wasn't even out in the west yet certainly turned a few heads. By the time Kingdom Hearts 3 was released, it spent more time explaining its plot threads than actually progressing the story.
KINGDOM HEARTS Union χ (keeping to the tradition of nonsensical sounding titles) is out right now for mobile devices and contains the same gacha-trappings one expects when dealing with a mobile game. Players obtain cards that they can use while traveling throughout Disney Worlds in the past during the Keyblade War. These plot tidbits happen between the forays into various Disney worlds such as Alice in Wonderland.
Adding another mobile title that hides away more plot elements is daring players to drop the series at this point. It's been said that "No one hates Kingdom Hearts more than Kingdom Hearts fans" due to the trope of adding plot thread after plot thread and then tangling them up for the next game to unravel and explain. To understand Kingdom Hearts you need a spreadsheet, and despite feedback from fans to simply make the games simple like the first title, when Kingdom Hearts 3 dropped it spent a large amount of its run-time with characters narrating the previous dozen or so titles in a decidedly monotone fashion.
Let's assume that all goes well and the upcoming mobile game is a success. Somehow its gameplay is stellar, its micro-transactions are passable enough to avoid scrutiny and it doesn't rehash what Union χ does. And let's say the new mobile title is not necessary to understand the next entry in the series. It still risks alienating its core base by overindulging in new game titles after expecting fans to play a few hundred hours to be fully versed and ready for Kingdom Hearts 3, as was the case with Union X.
Fans that prepared themselves for Kingdom Hearts 3 got exposition dumps regardless, including a time travel plotline, and those who only played the mainline titles probably didn't understand the exposition they were given.
There is also no reason to expect a competent storyline in this newest entry. Kingdom Hearts is held up by its character interactions, music, premise and gameplay, but its writing has always been a mixed bag. Sora is a lovable enough protagonist to get away with it, and being joined by Donald and Goofy is about as big a handicap as the series is going to get.
Considering the amount of rehashing that's been done across the series (visiting the same Disney worlds over and over) it's very likely the new title will simply have Xehanort's pawns revisit Agrabah and meet with Aladdin unless the title wants to A) drop the Disney worlds entirely, B) use new Disney movies or C) start implementing Final Fantasy worlds. As Kingdom Hearts has never used a Final Fantasy world, this means another mobile title will most likely be a rehash, and that's not about to excite anybody else. But it's very possible we're wrong on this-- time will tell, but we suggest sticking to mainline entries and toning down the exposition. Kingdom Hearts can still be a wonderful branch of escapism, but it needs a lot of work.