I have to disagree with other responses. I think you could make a career of this for the same reason management consultants can.
You could swoop in and "fix" some stuff and then leave the in-house team not understanding what you've done. That sounds profitable. You might even get called back to fix things a second time.
As with management consulting, I think it would at the system level tend to do more harm than good, even if you do good work and get paid well for it. I agree strongly with feoren that the code needs to reflect the in-house developers' mental models of the domain or everything will fall apart. If you fix things and then give them a bunch of processes and coding standards to follow, they will not do well and you will be thought of as some clueless architecture astronaut by them. But profitably.
You could swoop in and "fix" some stuff and then leave the in-house team not understanding what you've done. That sounds profitable. You might even get called back to fix things a second time.
As with management consulting, I think it would at the system level tend to do more harm than good, even if you do good work and get paid well for it. I agree strongly with feoren that the code needs to reflect the in-house developers' mental models of the domain or everything will fall apart. If you fix things and then give them a bunch of processes and coding standards to follow, they will not do well and you will be thought of as some clueless architecture astronaut by them. But profitably.