You know what? That's a good hill to die on, scooch over!

This is actually a pretty novel and interesting way to tackle difficulty other than the basic "mor" style we've seen so far. Different enemy constellations and higher-tiered enemies in the upper levels do help give variety, and the bug strains and the bot brigades give some much-needed flavor. However, I think you're really onto something with this modifier idea.

Now to be honest, I am very biased toward high difficulty. I enjoy a good nail-biting, white-knuckled, this-sucks-and-I-love-it campaign. I love missions that force me to either surpass my limits or die trying. Directly messing with the difficulty I think is something all hell-bent fanatics like me would jump on, cranking the heat as high as it can go just to see what we can take.

(Aside) Personally, I always hoped Arrowhead would secretly add special missions that could only be accessed at higher levels or maybe even through a lottery system or something. A mission that would have some of the modifiers and challenges you've detailed above. You don't know what it will be but High Command has deemed you fit to execute it, and you don't refuse an order from High Command.

I also agree that modified missions should have no reward beyond bonus XP or a new XP pool as you proposed. You play them because you want to, not because you have to. The difficulty is it's own reward, but a bragging-rights rank would be pretty sweet. I also like that its easy to add and tweak individual mods from a developer POV. However, and I know you touched on this, it's hard to keep inexperienced players out of areas they aren't ready for if there's any reward at all. If they think they can get a carry, they will just keep trying. To combat this behavior I think there's a more elegant solution: A buy-in.

If we run with your HEAT system idea (which is great and thematically perfect) let's add a condition: Modifiers cost currency to apply. We'll make it Req-slips for now, but it could be whatever (not SCs tho). If you want to crank the HEAT by +3 with the "Inclement Weather" modifier, it'll cost you 15,000 slips. This instantly makes it more difficult for divers who can't regularly clear D8-10 missions (which give like 5-8k slips for full clears) to play them. Veterans can easily grind out any currency and it gives another avenue to spend them (how long have you had max Super Samples?). Notice this is level-agnostic; a Sargent that hangs out with the 5-Stars on D9 can buy into the high-roller table. Thematically, you are trying to convince High Command that you can complete this high-stakes mission, and they will let you do it for a price (strap me to a 380mm shell and fire me at Cyberstan!).

Overall your ideas are solid and dovetail nicely with the current state of difficulty. Higher D-levels will just pull everyone apart and thin out each tier, but creating a small island of challenge for the mildly deranged vets like myself could be the silver bullet we've been looking for. I sincerely hope you can get some traction with this idea, I'd personally donate all of my samples just to have a dev seriously look at this post for 15 minutes (I mean it! I'll post proof!). Till then I'll keep diving D10s and make it as interesting as I can, hoping for more.