Adonalsium...
Alright, so we know that we have Dragonsteel coming- that's going to explain Hoid, the Shattering, Shards (and their holders) , etc. We know that Adonalsium is an incredibly powerful being, the likes of which have never been seen in the Cosmere, he's 16/maybe 17 shards all combined into one entity. This means practically endless power. If Adonalsium simply is a large amount of investiture, it would be incredibly difficult to kill him, as he would have nearly perfect shardaic foresight, probably seeing if any aluminum is coming near him. Aluminum would be a cheap way to take out a god, and I think we know Sanderson better than that. If he's going to kill a god, it's going to be incredibly epic, dramatic, and will probably take about half of a book to explain.