So osu! have this thing called "Game Modifiers" which changes gameplay in many ways. But the mods aren't flexible enough in my opinion. I think there should be a different Game Modifiers menu, somewhat similar to StepMania's gameplay options menu.

What it will look like

There will be sliders to change AR, OD, CS, HP and speed individually. This will replace mods that affect the mentioned specs(Easy, Half Time, Hard Rock, Double Time)
For visual mods(Hidden, Flashlight, Fade-in), they will be grouped in a "Visual" category.
For No Fail, it will be grouped under the "Fail/Retry When..." option, which I will talk about later on.
For Nightcore and Daycore, they will be grouped in a "Music Effects" option
For mods in Special category, they will stay there.

AR, OD, CS, HP and speed sliders

These sliders are independent and will have caps(0<=HP or OD<=10; 0<=AR<=11; 2<=CS<=7; 0.1<=speed<=(whatever caps))

Visual Mods

A switch to enable/disable Hidden, Flashlight, Fade-in. Hidden will turn off as soon as Fade-in is turned on and vice versa since Fade-in functions as the inverse of Hidden in osu!mania.

Music Effects

This will be similar to the Visual Mods group mentioned above. This will host all kinds of music effects (Bass-Boosted and stuff?) and will have cases like Hidden and Fade-in, where they are the inverses of each other.
Nightcore will lock the speed to 1.5x and Daycore will lock the speed to 0.5x
When you change the Speed slider with Nightcore/Daycore enabled it will be disabled instantly.

Special

This category will have smaller subcategories like Fun and Automated and stuff.

Scoring

Even though PP is now the ranking metric of the game, we still have ranked score and we still have score leaderboards. So how do you calculated score if everything is independent? My idea is that we use ScoreV2, but with the score cap based on the star rating of the game. The formula will look something like this:

scorev2-cap=star-rating-after-all-modifications*rate

with rate being whatever number the devs want to choose.

that could work but it might not be that user friendly. it would need community feedback to pick how that could be arranged. i think its a good idea but it would need alot of work.

McOsu! already features a lot of the stuff you mention. Check it out on steam if you want to (it's free and imports everything from osu!)

'Daycore' and 'Nightcore' are the pitch-changing relatives to 'Halftime' and 'Doubletime' but as their name suggests their values don't increase bpm by a multiplier of 0.5x resp. 2x but instead by 0.75x and 1.5x. Daycore exists in osu!Lazer already btw (not in any osu! version afaik).

Now, the fixed values for OD, CS, HP, AR and BPM are all possible to be changed by the user if it were to be allowed, but.
While AR might be the most subjectively hardest modifier to put a fixed value on it's still something the mapper chose to be just that exact value and I don't know if I like the fact that you could just neglect the difficulty in reading on for example an AR-8.5 165bpm alternate map, just by switching AR to something the user feels like is more comfortable. Maps with higher note-density surely will be easier to read when users can bump up that AR value as they wish but, is that really the point the mapper is trying to make?

(Might be able to split scoreboards into something like 'Original' and 'Modified' like supporters can split HR, DT etc. and people can still enjoy the mappers true intention)

Followed by AR is OD CS and HP..
CS for me is also kind of bundled with AR in having to do with note-density and gives off a special feel when it's either very high or very low and while it can be really challenging and rewarding whilst smaller, how many maps do really take advantage of this in the current state of osu where just about everything is cs 4-4.5 so people can still put HR on it (ofc there are some special cases where maps have like cs6-7 and oh boy am I impressed by people who play that shit). BUT yet again the fact that people can do amazing scores on cs7 would be kind of neglected for the same reason as with the AR I feel like.

I do like the idea of grouping modifiers but there aren't really that many as of right now (while lazer might introduce more) so the group 'Music Effects' will have to contain a lot more presets if this is to be added. Everything can still easily fit on one screen (without the 'sliders' that is).

The SCORING SYSTEM. YES. The neverending chase for a perfect scoring system.
Surely we could do some math to calculate the score for leaderboards based on the sliders multipliers that's not the hard part, PP isn't really either.
People would abuse the shit out of this though to farm even more score and the fight for #1 on some maps would never end cuz it'd just be "who can increase the sliders the most and still fc". As you mentioned there'd have to be a new way of calculating score for decimal changes in say, OD alone (but how do we fairly calculate these values).

I'm personally not a fan of these changes and I strongly feel like this has been discussed earlier so thread might die immediately 'cause 'closing thread due to duplicate'

This request is a bit of a mess. I suggest taking individual elements and talking about how their properties might be made modifiable. Here is a few examples:

DT/HT:

• Make into a speed modifier that controls how fast the map is sped up by. Controlled via a slider in the mod settings, it it would allow a player to set the map speed from x0.5 to x2.0. Most of the speeds should be unranked with 3 presets in place which are ranked and correspond to HT, no-mod, and DT.

Fade-In:

• Allow controllable Fade-In mod properties: how high the max faded area gets and what combo it takes to get to the max faded area.

Scoring:

• Allow the player to define their own custom scoring methods. This would allow the player to define score as an written expression and allows use of the following variables: misses, combo, acc, score_50, score_100, score_200, score_300, score_max, and many other variables. Any player defined scoring is unranked, and presets would include scorev1 and scorev2, with scorev1 being the only configuration ranked at this moment.