Not that high when it's all new characters. I think some characters more easily write themselves when they're new and the writers who are assigned to them have completely free reign to go where they please with them. In ME3 a big issue was the handing off old cast to new writers. As much as I liked Wrex in ME3 I could tell he had a new writer and it was also obvious that TIM, Udina and Anderson were not written by the same writers because one was almost nonsensical the entire time, the other lacked the caricatured speech and got the most undeserving death I have seen in a long time, and the third kept saying "Hell" and "god" all the time like a real marine when he used to be more... collected and formal.
To be honest ME3 was bad at holding things up to the standard. They cheapened the Geth, they cheapened EDI, Samara, Udina and TIM.
At the same time a character like Thane who was handed off to a new writer got exactly the closure I was hoping for right down to the smallest detail. I was very impressed and happy with that. Anderson was also still a very good character, it was just his character "voice" (writing term) and script that was too different and I didn't feel the ME1 or ME2 nostalgia I should've when I was talking to him.
I think there'll be a lot of he former staff on this one and I have faith that Schlerf can create some okay new characters even if they aren't funny (because his writing always comes off as "too serious" to me).
I think they all know what they're going into and like with all other games they've made I'm also sure they really fight for making the new cast original and unique while keeping the standard.
In terms of plot I have great fears especially when, in every interview, with Scherlf or even that Bioware blogpost he heavily emphasises that "characters > plot" and for H4 he outright says "I don't think there's much we can do with plot after H123". I think no matter what plot and charcters should always go 100% hand in hand for every good story. No overfocus on one or the other, just find a balance that feels good. I love characters. They are the driving force of storytelling but they can't be good without a good plot unless you want meandering.