By new characters, I suspect we’re talking about the existing Splash Mountain AAs in slightly different outfits. So possibly no Ray, no Facilier, not even the frog forms that the protagonists inhabit for most of the film. The sequel format is a bad idea, even worse then in Frozen Ever After.
There is certainly good reason to replace Splash with a Frog ride, but if they are going to do so Disney needs to level the entire structure and start from scratch. The fact that they aren’t going to do that is more proof, if any were needed, that current Disney design philosophy doesn’t understand theming - they see it as a bucket of paint that can be applied to any structure rather then something that needs to be wound into every step of the design process, from the placement of a ride to the nature of the physical ride experience to the shape of the structure. To see how little Disney understands this, we need only look at GotG - even among the many posters that enjoy the physical experience of the (very expensive) ride, the story and theme are secondary at best, badly implemented at worst - a thin veneer on a generic structure, so detached that they seem like they could be swapped out overnight. I love MMRR, but it’s placement and queue (up to the pre-show) is an uninspired mess slapped into an unsuitable space. Much of the spirit of Frog comes from the urban setting of New Orleans - where will that be in the ride? Is the Louisiana swamp known for its mountains?
The motives behind this change are good, but the half-hearted way Disney is approaching it reeks of insincerity. Go big. Rip it out and start again - with a massive budget.