I'm more of the go big or go home guy when doing crossovers. A Sith could show up a few years before TFE, scuffle with TLR when investigating the well, escape with his life, and report to the whole freaking Sith Empire! The Lord Ruler could easily hold his own against three or four Sith Lords at once, if he can push away their lightsabers.
I also think having Kelsier work as a Sith slave instead of in the pits would be great way for him to snap offworld, train as a mistborn alongside the Jedi, and fight to free Scadrial from the Sith; ending up as an ally of TLR against their common enemy.
...okay, I mostly only wonder what it would be like if Kel joind forces with TLR. Can you blame me?