are you looking for something challenging? How about a Saiga conversion, but not a 12.
And not just a PG conversion, but a real conversion, GB, FSB, russian filding stock the whole 'enchilada' so to speak. I am working on a 5.56 now, very challenging to say the least.
A Saiga in 410 or 308, I think I like the 410 because it would certainly be unique. And used by Russian security forces, a real in service type gun.
Your other choices would be great, the 105 needs that extended brake (unless SBR'd), but easily made from a 74m kit, the 74m is great too (I think mine is my favorite shooter now)
Of course the 12 conversion is cool, but the recoil makes it not fun to shoot unless you like bruises. I like the romak, but it's just too long for me.
Unless your itching to start another build right away you may wait a bit to see what the boats are bringing, global is supposed to have the tantals coming soon and I would imagine there will be other straglers coming in too.
Damn, I just can't stop this obession with these AK's