Seconding GURPS! I feel like there's not enough emphasis on diverging from the printed rules. If you're not sure which target number to use, or exactly what modifier to apply to it, just pick one and go! It won't break!
"When in doubt, roll and shout!" shows up a few times in the core books, but they really need to emphasize that idea more - people don't seem to have picked up on it.
I came in to say this! In addition, if you still have the bug to hack/mod whatever system you're playing, you don't have to explain the new rules to anyone else; you can change rules on the fly, or even rerun a scene a few different ways to see how things shake out. You can do this for a multiplayer game too (running everyone yourself); but in a solo game, you can try it as soon as you think of it!
Also, to expand slightly on GM Emulators - Mythic is the big one I know of, and they just released a second edition that's supposed to be rather good!
Hah, I'm an unabashed GURPS fanboy; I'm very familiar with the huge variety of modifiers! I actually find it less complicated than 5e, though - sure, there's a lot of options for modifiers; but if you need a modifier you don't know, you just eyeball it and narrate it. Look it up later if you want to check your work; but toss out anything you don't need.
There's an often-missed table describing Task Difficulty Modifiers in the core books (discussed more here: https://gurps.fandom.com/wiki/Task_Difficulty_Modifier) - this is the core of how I run GURPS. I figure out how hard I want a given task to be based on what's been described, narrate the situation, and give them a combined modifier based on what feels right. Climbing over a broken wall? Roll your Climbing skill. At night? Eh, roll at -2. We said it was drizzling earlier? Shift to -4. They're taking their time to go carefully, and risking discovery? Back to -2, and we'll roll to see if anyone comes along while they're climbing. Are those the exact right modifiers? I'unno, you want to crack open the books now, or find out what happens?
But, just to keep from overwhelming a new GM, I'd point them at GURPS Lite first - it should be able to handle most of Frostpunk okay, it's only 30ish pages, and it's free!
I'm only marginally familiar with Frostpunk as a genre - what sort of game is your friend looking for? Gritty apocalyptic survival with some settlement building aspects?
Ironsworn could be easily tweaked to fill that need, I think. I suspect Savage Worlds could too. I see a lot of suggestions at https://forum.rpg.net/index.php?threads/frostpunk-tabletop-system-recommendations.858901/ too.
If I may ask, why is GURPS not desirable? I know it has a few supplements for exactly this; including character templates to simplify chargen.
Ah, I forgot this when I posted earlier - https://1shotadventures.com/ has a series of free one-shot adventures for various systems, including a few solo adventures. Both solo adventures are D&D5E compatible!
One suggestion I've seen for dealing with fast one-shot kills like this is to treat it as "captured" - your wizard could have woken up a prisoner of the goblins, for example; and had to figure out how to escape! And maybe retrieve his gear. Someone might have saved you from the ravening beast, but now you owe them some kind of debt! Depending on the game style you want to play, you could still have a permanent change as a result of the "death" (from lost gear to scars to lost limbs!), but this would both make it more adventure-flavored rather than gritty, and more survivable.
In addition to the relationship dynamics everyone else is talking about, I'd like to point out the marketing problem - most tabletop games are sold in kid-friendly venues. Along with the much smaller market, such a producer would run into the same issue as other M-rated games: very few vendors would actually carry it.
Happy to! The non-carpet items I listed are actually examples listed in the spell description itself. If I was pushing the system, I'd want to try casting it on a riding golem, or a dancing shield! Or maybe just cast Initiative on the object as well to see if it could manage itself while I'm off it.
Slams have annoyed me in the past with another character - I had a Gargoyle shield-user for a while who could, in battles under open sky, use a dive to build up speed and basically snipe troublesome targets (particularly archers). Since he was built like a tank, it was rare for anything to be able to push back enough to even slow him down; but it was still a six-roll process every time to make sure of that. Once in a while he'd go after something strong enough it could push him back, but this was always pretty obvious before the dice started rolling.
...And now that I've typed it out, I realize that this sort of thing is what the Mook rules are for - if anything hits a mook, they're down, sidestepping the whole issue! Since that's the kind of target you'd usually want to target anyway for the thing keeping you in the air, I'm probably over-optimizing here.
Generally, I'm playing with one main character with some sidekicks, assistants, etc. These might stick around or rotate out as I play, depending on the narrative and how much fun I'm having with them. Sometimes, I'll have a few character concepts I can't pick between; in which case I'll usually end up running them as a party.
I've seen this a lot in discussion about the Eberron setting - the setting's author has a blog where he goes over his take on things; but he intentionally leaves a lot of things unanswered (including the cause of the biggest natural disaster in living memory). People often talk about things IME (In My Eberron) when detailing what happened in their games.
It's a refreshing take on an issue with a published setting.
I don't typically pay much attention to D&D specifically; but I do remember questions like this coming up on the solo roleplaying subreddit. I think there were a couple of tools that were recommended for that; I think "DM Yourself" was one of them?
I jumped in earlier on a GURPS comment, but I also want to call out Ironsworn and its sequel Starforged. They're built from the ground up to run GM-guided or GM-less, and include a lot of the work traditionally done by the GM, divvied up among the players and handled as part of play. You fly up to a settlement, and roll for more details about what you find there as you explore and interact with the locals. It takes a bit of a switch in thinking about how you run a game, but it's also helped improve the more traditional GM-driven games I run!
It's low-crunch, but I've meshed it with GURPS in a way that I'm pretty happy with too, for when I want a little more detail about what's going on!