TL;DR- I think Kelsier is a Spren, in a Nahel bond, and IS his eye spike (summoned as a spike a'la a shardblade). With that in mind, the metal of his spike would be a god metal, either Lerasium or more likely an Alloy of Lerasium and Atium either Harmonium or a different alloy of different percentages of the two.
Let me defend that a bit.
First, most everyone agrees that what Kelsier is/was at the end of SH, a bodyless cognitive shadow, is categorically the same type of being as a spren. We have WoBs that support this and taking it further we have WoBs that state he could enter a Nahel bond as well.
So its not a totally crazy idea.
Next I want to talk about who he might be bonded with. Spook. In SH we see the Connection between Kelsier and Spook. If they solidified this connection and progressed in that connection, I imagine it would be considered a Nahel bond and if it got to a certain strength I don't see why it wouldn't have similar capabilities to a radian bond in the form of summoning. Now this would also imply that Spook is still alive. Enter the Spook is 16 (from tSA) theory. This would give that theory the motivation for why he would be skipping forward in time and keeping himself alive. To keep Kelsier alive until another solution is found to keep him physical.
Overall, I think this theory could explain why Kels body is the way it is along with the problems he's having, I don't think its any more crazy then saying that he's pined to a body using a regular spike in a totally unique way, and I think it plays well with other fan favorite theories such as he has a Kandra body and 16 is Spook. Do I think its true? Na probably not, but I think it sounds cool and makes sense. I'd love to hear thoughts both for and against.
(This is my first post on here, please let me know if I've done anything wrong. Very excited to join this part of the community)