Well, this sort of idea came to mind, and I've been doing it to myself to help track my progress in the game and challenge myself to get better, but here's the idea:

Basically, a challenge marathon is a saved file, containing "trials" that users can create, and must stay specific to a certain game mode.
A trial is a round of the game mode, on a specified already ranked beatmap, on a certain difficulty, with given conditions. (Unapproved beatmaps are acceptable as long as they haven't been updated for a month or more.)
These conditions and consist of things like get an FC, score over 3,000,000 points, miss less than 5 times, don't fail, +Hidden +Hardrock, get a 200 combo, ect.
And then, the player who makes the challenge marathon can make the challenge range from the first trial being incredibly easy (Don't fail Bill Nye on Normal) to the final trial being supremely difficult (Get over 400 combo with +Hidden on Cruel Clocks)
If the player doesn't have the map necessary to play it, they can go to the website to download it if they need to, or download via osu!direct. If the map happens to be not ranked anymore or deleted, the trial is skipped altogether.
If the player completes the trial, they succeed to the next trial in the list.
The Challenge Marathon list maker can determine how many chances players get at completing each trial. Zero means infinite. 1 and up for how many chances.
All the information about each trial is saved into it's own file, which can be shared to others to enjoy! And others can try out your challenges and try to clear them all
I just thought this would be a rather simple way to share some fun challenges with fellow players, using what's already in osu!

sounds like "Nonstop and Endurance Mode" t/2902

Well, this differs. The request you posted specifies playing a random assortment of your songs nonstop.
The one I suggested involves playing specific beatmaps with specific conditions that are user-made lists, in hopes of having a target set of maps that will more so challenge the player's skill than test their endurance.
Regardless though, I support that other topic too.

eh, that's what I believe that request is about. It be better to have more options than to specify it to just "endurance", "Combos would carry over songs", etc. I find them to be the same general ideas. peppy choses how he want to implement it, so it doesn't really matter how much differently you specify your request.

Specific conditions? By that you mean "You could add mods to certain maps to give your course a different twist." What says in that request's OP. Isn't it obvious that you would be able to specify which beatmaps you want to play in that request? This is already exactly what you're suggesting:

Kitokofox wrote:

The one I suggested involves playing specific beatmaps with specific conditions that are user-made lists

Well, in Nonstop mode the objective is simply "get as much score as possible while keeping yourself alive". In this mode the objective is "complete the given objective for each song; get as many objectives as possible while not using too many chances". I can see how the ideas differ.

Do you know some "Mission Mode" or "Challenge Mode" in some games? I'll use Sonic the Hedgehog because it's the first thing that comes to my mind.
In some StH game, a stage's usual objective is "reach the finish", much like osu!. However, there are generally some missions that can be added in Mission Mode, such as "get 200 Rings in 60 seconds", "reach the finish within 120 seconds", etc. In osu!, this can be "score at least 3m", "get a 200 combo", etc.

A Nonstop mode equivalent in that game is basically normal stages played in order/random; play as far as possible and score as much as possible.
A Challenge Marathons equivalent in that game is basically playing as many missions as possible without failing too many times.

...well, that doesn't seem to be any better. Whatever. The point is that this is not a duplicate of that, and the underlying ideas actually differ.

The problem with this is that there are just so many possibilities of missions. If peppy wants to add this, peppy needs to limit the possibilities somewhat (for example only "score something", "reach combo something", and "don't miss too many times" available first), and adding more over time. But this would be really cumbersome.

This idea is very interesting, but I doubt peppy wants to implement it. Whatever, still support from me.

I thank you for helping differentiate this request from the other.
I have a hard time explaining things sometimes, and you helped a lot!
I too play the Sonic the Hedgehog series, and this was somewhat inspired by that.

and I know, the many possibilities of missions lets everyone be free to make easy or painstakingly difficult missions.