I do have a lot to say about this cube, as I have wanted to delve into the territory of constructing a cube with multiple copy cards, though I haven't gotten around to it yet because to me it seems very difficult when you're adding an extra dimension to design.
Personally my favorite perk to having multiple copies of cards is that it becomes easier to draft and play cards that synergize with types and attributes, as you have also realized. As an extension I'd love to see the four Elemental Lords integrated, but it might be more difficult to resolve some than others. They might be too powerful, but they seem challenging enough to summon that their payoff is worth it.
There are a lot of niche spells and traps that have remained in my cube that could fit perfectly in here. I'd have to go back in check, but for one I think Punch in the Box fits quite well in here.
Also, do you think Kozmoll Wickedwitch is a bit too powerful in this cube? It's a 1900 beater that also can't die to card effects/battle, so the only way around it is to stick a bigger monster on the board or negate its effect. It's notoriously strong in my cubes but I guess there's more effect negation in here, so it might be a bit weaker.
EDIT: The Mecha Phantom Beasts are a pretty good engine, multiple Megaraptors Tetherwolfs Blackfalcons etc, some might be stronger than others though.
Another cool engine would be Jurrac Guiaba, Velo, and Dino. Dino is crazy in cube and the other two are pretty good card advantage wise. Evilswarms are a bit of a stretch, but a lot of them are decent on their own and when you draft copies of Kerykeion you can get some good value.