Ok.
For the love of God, no.
To start off, Donald and Goofy will not be properly, eternally replaced. Much like in The World that Never Was, we might get K and R as temporary party members, or switchable, but not as replacements for Donald and Goofy.
Seeing how, ya know, even Riku being a party member in TWTNW isn't really 'canon'. The developers added it without Nomura telling them to do so. I think it comes to show about Nomura's intentions to put even Riku in as a party member aside from special boss events.
That done and over with, we have the issue of Riku as a character being a team player. He didn't stick around even with Mickey for too long, and handled himself fine battle wise without Mickey for most of R-R. If he can, he seems more like the type to move on his own, rather than stick around with Sora. Not so much because he doesn't trust Sora, but because if he could spare Sora the trouble and the grief, he'd do it on his own.
And then we have Kairi. Who officially is not in a possession of a Keyblade. I think it's safe to say that with Riku being the one that pulled the Keyblade she used out of thin air, that it's Riku's Keyblade. He summoned, his Keyblade. End of Story.
As to why he had it, that's going into Duel Wielding territory, but if we say it's because of XH - poor Kairi isn't going to have a Keyblade for Riku to give her, seeing how if XH is gone - and he's gone - so is the Keyblade Riku managed to Duel Wield thanks to him (much like how Ven is how Roxas and Sora managed to wield at the same time - Sora got Riku's at the start of KH1, and Roxas got Ven's when he was born). So that leaves Kairi with undeveloped, never explored powers of light. Now, she MIGHT become like Minnie, or FFVII's Aerith, but if they know what's good for them (and sadly they don't, more often than not, when it comes to Kairi) they won't have her as a constant party member right from the start. And that is of course aside from the fact that with BBS, the PoHs are no doubt bound to become plot devices. She won't have the screen time to waste being a party member.
All in all, it sums up to what KH2 kept hammering into our heads.
Sora, Donald, and Goofy.
Smile has Spoken.