Not sure what you mean. Did you read it? x/x.go is a file containing utility functions, equivalent of utils package.
x.Trace is for RPC tracing, which attaches debugging information across every rpc. log.Fatal or other log commands are printed on the console – we’ve mostly moved away from those, and towards x.Trace. This is equivalent of /rpcz at Google. So, we’d still need that.
I think what you’re proposing is to create a new module called derr. That’s okay, but consider using x package which already exists for this purpose.
I suspect what you mean is: derr.C(cond), where cond has to be true, otherwise we log fatal. The rest sound okay. Note that most errors via log probably are log fatal, unless they’re culminating to the end user – because we generally use tracing.