(I'm new to this, so there are spoilers for the whole Cosmere up to ROW - not sure if I need to put that here, but hopefully it's just a amateur mistake)
So, given the list of shards that we know of, we are missing two in the Cosmere. I'm sure we will find out the identity of the other two, but I'd like to put forth a shard that should be there:
Logic
Sure, it's not the sexy type of emotion, but I think it's one that should exist and may be the one that Wit (or someone else, feel free to correct me b/c I can't remember fully who said it) was referring to.
Who better to inhabit this shard than Jasnah herself. Who better to keep around as maybe a mitigating factor in the future of the cosmere or someone who brings the shards back together again (which I think is the end goal of the Cosmere, but that's just spitballing by me). I also think this is one of Wit's many goals is to reform shards with people across the Cosmere he trusts. I think he didn't trust some of the vessels in the past, so he's looking to stack the shards with people who he can rely on like Sazed.
Also, If Wit is going to be a main character in Misborn Era 4 (which I believe will be the Cosmere era) , he may have a goal of uniting adonalsium and reuniting with his long lost love? I mean if you can snag Wit, you've got to be extraordinary and Jasnah fits the bill.