Cross over LBT fanfics is a real no-no for me. No other thing can be introduced into an LBT environment that already exists (like star wars or other tv show or book). If you add your own characters that you made up on your own, then that's okay, but I just hate it when people put a couple of shows together. It may start out okay, but then the writers find themselves desperately trying to improvise and put things together and it just turns out rediculous (like one I saw Littlefoot was battling Luke Skywalker in a lightsaber duel after turning to the dark side.) Though these things may be sort of funny, people tend to get obsessed with them, degrading and humiliating the original movies of both sides in their stories and eventually losing the original point and plot of the individual movies they feature in their fanfic. Quite sad.
Sexual things in a fanfic are okay so long as they're tasteful, not too explicit, and don't involve two creatures from different movies or books. By not too explicit, I mean very mild things that make you assume sexual things are going on between the characters, not mild sex scenes! It takes a lot of skill to make the reader think that sexual things are going on between the characters without just coming out and saying it. They usually go something like this:
"I want to...........slksdjflkjgsad..sdf.dshfolshjlkijdskglj.l.jgslkdlkjlkgjdslkgj and sdflkdsjfklslkdjljlksdf...sgjlsfj..."
That's not what I'm talking about. I'm not talking about literally speaking what they're going to do out of the book. Just little hints. It usually takes a long time to make the reader think such things without just saying it or just turning it into something completely nasty. That's why I usually say "No sexual relations between characters in fanfics". I'm not opposed to it, but it's just too hard to include those things without including them or saying things about them for most people.