As a note, since this is going to be a big budget theatrical movie we can expect a lot of big stars, either new to the franchise or returning voice actors.
Its highly likely that james earl jones and ernie sabella will return. Madge Sinclair and Zoe Leader won't be returning because they are dead, and it is very unlikely that niketa calame and jonathan taylor thomas will reprise their roles as young simba and nala.
I predict that lupita n'yongo and chadwick bosman will be in it. Both are comfy-cozy with Disney right now. If I had to have a replacement for James Earl Jones, maybe Denzel Washington?
Maybe Samuel L Jackson too? He was in The Incredibles, and he seems to be on good terms with disney as well.
I would like Lupita as Adult Nala and Chadwick as Zazu. If they don't get back Nathan Lane, Sam can be Timon.