We've already seen that there is some room to reduce the weight further. A small lens like this with electronic fly-by-wire focus, etc. could potentially be made in this mount (although it would be much more appealing in EF mount) and IS inclusion would be a big improvement. But all that is just shooting in pitch darkness.Why? I don't see any advantage to refreshing it for EOS-R over EOS.
But a 400/2.8 is totally illogical, the new one will be perfectly good for at least the next 7 years in EF-mount only with new EF bodies behind it.