I had started a Castle-L&C crossover story a couple of years back but never finished it. The series had just taken a dramatic turn and I was left going into a summer with the 'return' of the partnership up in the air as to the direction it would take.
Now things have settled into a more accessible pattern and I might be tempted to give it another try. (Only my incredible laziness when it comes to writing stands in the way).
I think Castle in many respects is what we all wished Lois and Clark could have been. Both series did/do the relationship (or the 'B' plots) aspects of the shows very well. But I have to give it to the writers of Castle when it comes to the main storyline. The crime plot has always been engaging and entertaining. Whereas sometimes the main storyline on Lois and Clark could get a bit silly (even allowing for the fantasy aspect of the show).
The biggest hurdle to writing a crossover is my own competence. I 'know' Lois Lane. I get her, I've had decades of experience with Lois Lane at many levels. I not sure I can be that comfortable with Kate Beckett. She has a lot of 'Lois Lane' in her but she's not just a cop version of Lois and I wouldn't want to portray her that way.
I'll have to mull this over for a while and see if I've got one more left in me.
Tank (who is currently indulging himself in the DVD sets frome the Castle series which he recently acquired... except for season one... that one has escaped him for the time being)
