In thinking about what would be necessary to turn Cold Iron into a complete game, one thing that is definitely needed is a decent bestiary, along with some hints at how to set up encounters (which of course also includes what sorts of treasure to give).
I was looking over the stuff that was made available to me, and realized it's pretty scanty. There is a series of summoning spells which provide stats for wolves, hawks, small lions, snakes, bears, large lions, and horses. The player who wrote up all the spells and such also provided a several page description of how to create monsters. That includes complete stats for dragons, wyverns, and gnaths (some kind of nasty feline - with some comment about "Who ever heard of a 1st level gnath?"). That document also provides some hints on hydras (with an admission that the GMs he talked to probably do them wrong). He also mentioned he had a huge specials chart for undead, but didn't want to share it. A bit of information (mostly mental attribute ranges back tracked from saves) is provided for goblins, ogres, cave trolls, stone trolls, flesh trolls, ghouls, displacer beasts, griffons, manticores, werewolves, orcs, urak hai, and moties (from Niven's A Mote in God's Eye). The Animate Dead spell indicates that undead have higher Strength and Constitution and lower Dexterity than when alive.
What the monster writeup stuff does do is provide some general hints. It talks about the square cube law, and provides a chart to match Strength and Size (and mentions that Constitution usually is the same as Size). It provides an extented attribute chart (with Strengths and Sizes high enough for anything you'd ever want to stat up - up to a size of 12.5 tons). Another chart shows how armor protection (T) and critical protection are related.
There is an admonishment that unless you want to run a killer world, keep monster levels low (at least for big monsters).
Somehow with all of this, and a bit of talking to the other GMs, I figured out how to run monsters. Of course looking back at my notes, I obvioulsy ran some things by the seat of my pants. There's no indication in my notes if ghoul paralysis has a penalty to saves or not (or even how long it lasts). Somewhere I got the idea that the way to handle skeletons is that you have to deal them 20 points of damage (past armor) in a single blow, otherwise nothing happened to them (you just chipped off some unimportant bones).
What I tended to use the most were goblins, trolls (really ogres, not dumb creatures like in D&D, and no regeneration - they may have been influenced by Harn which is the setting I used for my first campaign), ghouls, zombies, skeletons, wights, spectres (corporeal, just the name for a spell casting wight), and an occaisional dragon or so. I have a module writeup with gnolls. I used a few animals (wolves and large felines mostly). I also had flesh trolls (the stupid, regenerating D&D kind). I know I used a very occaisional dragon or wyvern, and I kind of vaguely recall a manticore. Oh, I had giant ants also. In general, most encounters were with intelligent humanoids (lots of goblin and troll encounters).
As to treasure, well that I really had to come up with my own. I think mostly I started off cautiously. I'd usually give decent opponents decent armor and quality weapons. The captain might get magic weapons and armor. I'd scatter a few potions and charged items around.
Hmm, looking at the gnoll module writeup:
The 6th level gnoll leader had +3 weapon and shield, 3 somewhat used charged items, 4 pretty good cure potions, and 4 buffing potions. He only had quality arrows and non-magical plate. The 3rd level troopers had quality weapon and shield, 2 decent cure potions, and one very used charged item, they had plate armor. There was an ogre zombie with +2 equipment and a wight with +2 equipment. The PC fighters were probably 8th level or so (not sure how close to the end of the campaign this was, the one PC fighter I have a character sheet for made 10th level, the mage made 6th level - but that fighter may have played several months longer).
In my first campaign, I have no PC character sheets from, but the main NPC made 12th level fighter/7th level cleric. There was some extended play with just one player. I remember fragments of a couple adventures. One was the great dragon slaying, where the NPC and the PC (and maybe another NPC or two) were exploring a cave, noticed a dragon, retreated to buff up and charged the dragon - the NPC rolled a many 9s attack and killed the dragon in one blow). I also remember an adventure where they attacked a necromancer in his caverns. He must have been a 9th level caster because every single undead in his retinue had an anti-magic shell charged item he had made for them (boy were the PCs annoyed - of course new players won't have this kind of joy since anti-magic shell is gone).
I think it would be nice to come up with a challenge rating sort of system. With a combat simulator, it would be possible to see how various creatures rated out against a "standard fighter". Spell casters would be harder, but if you know how the fighters rate out, and you match their challenge, giving the bad guys a spell caster or two at the same level as the PCs would be really tough, a higher level would be unwise, and a level or two lower should give the PCs a good fight. Obviously a hard to hurt spell caster (like a troll) would rate a little higher than a wimp (like a goblin).
What might also be interesting to work out (and should fall out of the challenge rating determination pretty easily) is a "level adjustment" system. I don't absolutely feel like anything the GM can throw at the PCs should be open for play, but it would be cool to know that it would be ok to let a PC run a troll as long as he was treated as if he was 2 levels higher for experience (or whatever the adjustment came out to). The only problem is that it might not be constant, but you might still be able to come up with a formula (perhaps a troll is equivalent 1.5xL+1, so a 6th level troll is comparable to a 10th level fighter). Then you just create a new XP chart for trolls. Monsters with specials would be harder to deal with, but I have used relatively few specials in Cold Iron (flesh trolls regenerate, ghouls paralyze, most undead see invisible, some undead only take fatigue/subdual damage from non-magical weapons, undead otherwise don't fatigue, dragons have breath weapons, basilisks petrify, rust monsters rust, blur wolves are harder to hit (unless you see invisible), shadow cats are surrounded by a shadow spell (need see invisible), some creatures have poison, and a few more). Humanoids who use weapons rarely have much in the way of specials (and I don't think it's too much to ask players not to run undead, and non-humanoids).
When writing up PC races, I keep away from making races that are clearly better spell casters than humans (though some races might have a better willpower which is important to spell casters offensively as well as defensively). I also make PC races trade off between Strength and Dexterity so no race is clearly better than humans there. I keep special abilities minor (night vision, infravision, leather skin [which stacks with armor], bonus to saves, need less sleep). Now players tend to take non-humans over humans, and that's ok, but they should be trading off at least something.
One of the things that I really like about Cold Iron is how easy it is to write up a creature. You pick a Size (or Strength and backtrack Size from Strength), that gives you Strength and Constitution. You pick a Dexterity (and most large creatures have some room to grow their Dexterity). Then a relatively simple chart gives you the base abilities by level (and I now have a spreadsheet which makes it real easy). When I'm in good form with the system, I can create a new monster in half an hour or less, and since I write up a progression for the monster from 1st level to 6th or sometimes 8th level (most animals and beasts only go up to 6th level), I have an instant variety of monsters for different power levels.
But the big question is how to turn this all into something someone else could work with. Especially someone who had no experience with the system at all.