Honestly. I think this will probably stay with Witness mainly because it stands out from the other channels.
Despite it having the same logo and promo style, the idents are what makes the channel stand out. So I'd say, let each channel look corporate but on screen, give them their own unique image.
A bit like the BBC's channels in the late '90s and early 2000s, then.
"Channel 5! The channel that brings you England goals!" -- Jonathan Pearce, 31 May 1997