If your replicating in the wild your basically stuck with organic chemistry as building blocks (C,N,H,O) plus a few 'exotics' when you really need them. Add to that the need for a plentiful energy source to replicate rapidly. We may be able to improve on some things and make a better form of chlorophyll etc., but as long as we are using organic chemistry it's going to be very hard to create something that does not quickly end up as something else’s food or host.