The problem with running a character on a carnosaur is that he can't join units, meaning he's open to getting sniped, and unless you get him charging something at the same time or after another unit, he's always gonna be suck fighting through steadfast and having to accept challenges that can keep him away from the characters he wants to kill (plus fighting a unit champion in a challenge usually limits his max wounds caused to six, which is way less then what he should be capable of).
But if you want to run a carnosaur then consider:
- Putting on Old Blood on him saves hero points for a scar blood BSB and skink priests.
- Putting a BSB scar vet on him gives you 18" of rerolls instead of 12"
- Putting your general on a carnosaur gives you 18" of inspiring presence, which is can be good for mobile armies using terradons or ripperdactals as they can really use the extra leadership. As to using a scar vet or an old-blood for this, the choice is up to you and depends on what else you want in your army.