Hang on- are you talking about Kelsier bonding a spren.... or are you talking about Kelsier being the 'spren' part of a bond with someone else? Cause it's cool to think about Kelsier bonding a spren like @Rune was describing. But what happens if someone is able to bond Kelsier (given that he is a spren-like being, a cognitive shadow that has been around for ages, becoming more like spren each day...)? I don't know what that looks like or if it's even possible but I like to think that it would be pretty awesome. Maybe an unchained bondsmith like Dalinar could make it happen somehow?
Maybe his mistborn abilities show through in that person, and his "survive" mandate / ideal would pass along some abilities that feel a little more in line with 'knights radiant'-type abilities...?
To answer your question, though - I think Kelsier bonded Szeth that could be pretty cool. Imagine Szeth trying to get both Nightblood and Kelsier's voices out of his head, telling him to destroy evil and kill the rich people. Adolin would be interesting as well (Adolin would help temper some of Kelsier's more murderous tendencies... but then again Kelsier would likely encourage some of Adolin's murderous tendencies). I would definitely not want it to be Hoid but I think Hoid wouldn't really want that either... Who else would be a terrible option?
CognitiveShadow's post in Bonding cognitive shadows (again) was marked as the answer