Looking through the script files, there's only a separate entry for dark frog blood with Acidity = 100. Presumably light frogs use normal blood.
However, eating a frog won't necessarily kill you. It does lower your INT, WIS and I think CHA though.

Not always though. IIRC this is also true for thrones and beds.

As for having couches turn hostile when you attack them, I don't know whether there's any existing code we can use for that. Using the ghost spawn code from coffins may work, but then the problem is whether we can specify an exact spawn position for the couch monster - so the destruction of the couch spawns a couch monster in the same position, so it appears you've angered the couch when really it's just spawned a monster on top of it.

It does indeed sound like a lot of this could be easily done with copying of code.

With respect to the healing/harmful gas, I think that could even be done with just a quick edit of the script files.
For example:

  Config HEALING_VAPOUR;
  {
    Color = rgb16(200, 60, 60);
    NameStem = "regenerative gas";
    Alpha = 200;
    AttachedGod = SOPHOS;
    Effect = EFFECT_HEAL;
    BreatheMessage = "Breathing this gas makes you feel refreshed!";
  }

In fact we already have mustard gas as the template for harmful gas, which could be simply copied and re-worded for fire gas. Otherwise we'd have to program in a new status effect for "on fire" rather than recycling what I assume is the acid code.
Also I am of the opinion we need more gas emitting monsters if we're going to be adding more gasses. Obviously they can be based off the skunk or magic mushroom code. An example could be healer monsters that use short-lived healing clouds to heal larger groups of monsters - giving you the opportunity to reap the benefits of these clouds too.

On the lines of artifact armors, I feel like cursed items like those in so many other roguelikes would be a really good fit for IVAN. But the other suggestions are pretty much covered outside of unique armor pieces (Boots of Strength, Helm of Brilliance etc.), so any artifact armor would have to be really unique, like the ridiculous shirt of the golden eagle. The shirt of the jaguar in LIVAN and the Decos Banana Co. shirts are also good examples.
I'd also like to see some less-practical effects or trade-off armor pieces like one that gives you ESP in exchange for making you absolutely blind, for example.

For safes, we already have chests and strongboxes made of different materials, and hidden vaults for said containers. On the other hand, we do not have mimics.
But I do like the idea of having the chance to find an item inside objects that you wouldn't normally break.

I'd be glad to help in any way I can. It'd be a good opportunity for me to practice my programming outside of a familiar language.

It's certainly not that useful once enemies start getting better at decapitating you or teleporting it off you...

This is what I was getting at. I consider myself lucky to find any sort of fire resistance items before reaching GC, and wielding Turox with your (likely) low tier cloth armor isn't going to end well for anyone.

I've had kamikaze dwarves spawn in UT2 once. That was a horrible (and short) experience. Now I don't eat flaming bananas or school food until I know I can handle whatever my newly boosted stats spawn.

Yeah, don't go wielding that Turox you found in UT2 straight away...

I had one game where there were literally enough cloaks to clothe the entire population of IVAN planet.
But could I find body armor? No.
Somehow survived long enough to get butchered by Rondol who really had no reason to show up other than to spite me.

Batman's post about the minefield reminded me of something.

Are the item spawn ratios randomized differently for each new character, much like how every different character has different training rates for his stats?

They're also really good for getting into that one room with the chest protected by meteoric steel walls.
Other than that, chucking a couple on top of a live mine makes it just that little bit more dangerous for whatever ambles into it.

You're not the only one. I've noticed patterns where the game likes to spawn lots of certain items too.
Good examples have been Armors of Great Health, cans of school food (great for END), and troll hide capes.

I'm not entirely sure whether these were caused by some coding in the game or just random strokes of luck though.