Here are my rules for this team:
Why such restrictive rules? In general, because each restriction also covers the cases without it. If I can do this without mods (like ladder runewords), I could in principle do it with them. If I can do it with limited item carrying capacity, I could do it unlimited. If I can reach seven Guardians without respecs or twinking items to younger characters, I could do the same with respecs or twinking as well. Any looser restriction would drive me to prove it over again with a tighter constraint, so this cuts past that slippery slope right to that endpoint.
And I'll cover some of the rules specifically in more detail. No mods is a hard-line starting point, which I'm absolutely sticking with. Once you start modding anything at all, it's a slippery slope to tweak and customize everything you want until the game barely resembles the original at all. I take this hard line with all the other writings you see on this site as well, the Civ series and FTL and the others. Any mod is not the original game; only a fully unmodified game truly represents the standard of playing the original game as it is rather than some individual wishing of what it might be.
I'm also fine to go without the common particulars of would-be mods. This includes no uber bosses offline, which I'm entirely okay with, keep the focus on the real endpoint of Guardianship. And this means no ladder runewords, which is a play style I also very much prefer. Because this forces you away from handing every single character and merc a one-right-choice Spirit or Insight runeword, but rather makes room for more varied options.
No additional stash space or mule characters. The stash is the size it is for good reason. Back during the game's heyday and active patching, Blizzard consistently rebuffed requests for more stash space, with the directive that the game isn't meant for accumulating infinite treasure forever, but you are meant to make choices about what your characters will continue to carry. And I want my game to operate that way. I want to be forced to get rid of marginal stuff, or else I'll end up with hundreds of choices for each equipment slot and take weeks to sort through them constantly.
No respecs. This is my rationale: respecs erode and eliminate the distinctions between builds. There is no such thing as reaching Guardian with different builds of the same class like a hammerdin or zealot or vengeadin, since any character could just do it with that class's most powerful build and then respec afterwards. I want to preserve the diversity and distinction of different ways to build characters. This includes having to get by the early acts on limited and makeshift skills and equipment -- figuring out all of that can actually quite be one of the more interesting areas of the game, that gets lost under most modern play styles.
No farming/repeating/rerunning areas or bosses. This is also to avoid a slippery slope. Without this rule, the right thing to do would be to farm endlessly in easy areas until all the equipment is as perfect as possible before moving on. I'd spend weeks at each such breakpoint like Mephisto on each difficulty or the Countess. I need to be moving onwards continuously. This is also the right motivation to clear areas and play through the game properly, to extract the maximum benefit from a single pass, rather than slip into the temptation of skipping and teleporting past anything inconvenient.
And finally, character death. That's an issue for any hardcore project, of course. I play hardcore because that's the only way that winning means something. There is no victory in softcore because there is no way to lose it. But a Hardcore Guardian is always an accomplishment. Sadly, however, a character death would upset the symmetry of a team project. The obvious answer is to rebuild a new character of the same class if someone dies... but I'm not thrilled with that option, that's actually a secret motivation to get someone killed, as a hidden way to respec and to run the replacement through an extra series of acts for more equipment. Upon starting this project, I haven't made a final decision of how to handle character death; I'll see if or when it happens and under what circumstances, and make the decisions then.