Time for a trickier one - here's a pack from Gaslight, Gatekeep, Green, my contest cube with all of the Forest duals in the basic land box and 30 copies of Land Grant.
I would pick Fire Covenant. Looking at your list there are a lot of creatures with power 2 or less, making it fairly cheap in terms of life cost for an instant speed, one-sided wrath. The only mark against it is that it is the only true gold card in the pack. I wonder if it might be too good for this cube despite that.Mondschwein, the "joke" behind the cube is that Green is effectively colorless because all of the mana sources except for one are either Forests (because Land Grant) or are able to produce (in other words, something like Dromoka's Command is effectively mono-white - one of the exciting parts of putting together the cube was that I could throw in a ton of Gx gold cards without having to worry about them taking up limited gold slots). There are also enough copies of Land Grant that everyone in the four-person draft is going to end up with 6-8 copies.
Looking at your Cube Cobra list helped me get the joke behind the cube. At first I couldn't understand why Selesnya cards were mixed in with the white cards, then I got what was happening. The project is intriguing, but I'm not sure whether green mana symbols in the costs are just noise that my brain has to learn to ignore when I am appraising cards. Given that being a gold card is treated as an additional cost by WotC play design, won't cards tend to be more powerful than cards? Other cards seem to get worse, for example Thirst for Discovery will always require you to discard two cards (unless duals are treated as basic lands in this cube).
Drafting any new cube requires you to appraise how powerful cards are in the context of what they can be drafted with. This cube also requires me to reappraise the value of gold cards with altered casting cost. I think I would need to draft it to find out whether it is a fun puzzle or just makes my brain hurt.