Really not sure what to make of the 'Joan Watson'/Lucy Liu thing - I guess it could make it sufficiently different to not be a Sherlock copy.
But if she does her usual thing (where she basically plays the same role in whatever she's in), then the BBC will hardly need worry about the competition!
I'm sort of philosophical about the situation - BBC don't have a monopoly on Sherlock Holmes, and given there are many versions of Victorian era Sherlock, there may be more than one modern version too.
Bit weirded out by JLM being offered, and taking on the role - but we don't know if he and BC have talked about it.
This is the first time Sherlock has been more popular with the ladies than with the men, thanks to the things BC has uniquely brought to the part. Having a 'rival' won't make the bbc version less well-written, or BC less compelling to watch. I think JLM is a good actor, but BC is compulsive, addictive viewing.
Also the formula of great part, well-written scripts, decent supporting characters, 'chemistry', and fine acting isn't that easy to emulate. Also there's the musical scoring...it's all brilliant.
Others can jump on the bandwagon all they like, but quality shows aren't just about mucking around with a great concept, throwing in some surprise casting choices and hoping for the best. Good luck to them, but it would have to be amazingly good...imho.