Given that I'm going to be doing more than ultramarines, and I've got two more marines that are mostly made of metal that I need to deal with (not counting Cypher Mk2), I'm thinking I'd like to get those dealt with first.
One has a helmet, so he could be anything, like the 5 other suspiciously light marines I have, which all also have helmets (three of which will be ultramarines, as noted).
This leaves me this last mostly metal marine (again, not counting Cypher Mk2, when I eventually get around to him), to do something with.
I have selected this pile of bits to come up with an idea.

Now I could be boring, and give him a plasma pistol and power sword, and try to come up with another possible chapter for him (not Ultramarines, Imperial Fists, Crimson Fists, or Blood Ravens, as those are what all the other Not Cypher marines will be).
Alternatively, I could build him with some random crazy gun (possibly including that spare giant gun from the riptide, because space marines and giant whirling cannons sound fun), and either create my own chapter for him, or maybe paint him up as a Reviler - those are the ones that keep attracting the attention of the Inquisition, right? Or is it Relictors?
E: it's the Relictors, and they are officially Excommunicate Traitoris.
Another alternative would be a nicely battered Lamenter, which I think is what I will go with, maybe.
Probably toting that giant cannon, because why not.
No point having a giant cannon if you're not going to use it, right?