I'm sure I'm not the only person who mixes stuff from different settings together. I have a lot of fun doing so. And it lets me use some really cool stuff that other people have thought of.
Things I add to Forgotten Realms:
1. Shifters from Eberron. I really like this race. I wish they weren't so screwed over with feats. I really need to do a conversion of them for S&W.
2. Broo from Glorantha. Perfect bad guys. If you aren't familiar with them, do a google search and read a couple articles, especially those with a focus on their god, Thed.
3. The Scarlet Brotherhood. Cool monk bad guys, who have a secret base in Chult in my world.
4. The City State of the Invincible Overlord, located in either the Border Kingdoms or the Western Heartlands, depending on convenience.
5. Various Planescape products. For when players want to explore extra dimensions.
6. Ravenloft monsters. The three monster books are just full of cool, creepy stuff.
7. The Ancients. An elven street gang from Shadowrun that I use in Waterdeep (minus motorcycles).
8. Adventures and maps. Just a grab bag of stuff here that I plop down whenever convenient. Which is kinda what these types of products are for, anyways.