Personally, I don't see why Kingdom Hearts couldn't take a big turn and include all Square Enix IPs in further entries.
A new lead is certainly possible, although I could see Riku possibly taking the helm of the new saga. There's a lot of charisma to be shown in him -- he's been very serious about his role amongst the Lights, Darknesses, and what have you. I could very much see him either carrying on the series or becoming a legacy character along with the rest of the cast from the Xehanort Arc.
Although the best chance of what's to come is that KH will no longer be a FF/Disney exclusive collaboration, and, truthfully, it hasn't been since KH3D. I'd love to see anyone from the Chrono series, Xenogears, Nier, or even Dragon Quest. (Although personally I'd like to see characters from more obscure FF games, like Tactics.)
Either way, we're not going to lose any characters. We're only going to gain more.