According to latest rumors on CR, Canon may announce next generation of C300 Cinema EOS cameras in late 2019. This means we may see Canon EOS C300 Mark III to be announced in late 2019. Currently this camera is already in testing.
Canon EOS C300 Mark III will feature with 4K video, but will be able to shoot 8K video with an upgrade for an additional cost after purchase. No detail yet how this will work. Other features will include 4.4.4 as well as a new electronic image stabilization