No one should "replace" Brass because Brass is irreplaceable. If I were still going to watch, I'd rather see a rotation of cop du jours, which I suspect they will do. This current crop of writers doesn't know how to write detectives like the old writers did (see Brass' reduced interaction with the CSIs as evidence). Actor who plays Crawford seems to be one of McGill's favorites so I expect he'll be recurring but I doubt he'll get a spot in the credits as a series regular. The whole point of not renewing Paul's contract was cutting costs so why add more? And really, do you want to see Vartann back? They'd probably just use him as a plot device like they did Vega and kill him off during sweeps.
Btw, not renewing a contract is not the same as being fired. Fired implies Paul did something wrong and he did nothing wrong. They just didn't renew his contract. That said, I hated the way Paul was told, the lame-ass reason they gave for not renewing his contract, but mostly I hated the way it ended for Brass. At least give him a proper send-off at some point in S15 and not just some passing "Where's Brass? Oh didn't you hear, he retired." bullshit.