I've noticed that osu!mania doesn't have a lot of mods in it, and I thought I'd come up with some mods (this MAY require hiding them or adding a extra mod page)
So here are some good ones:

Speed Rate (2.0x, 3.0x, C250, .etc)
These increase the speed of how the fast the notes come at you. I personally would use this instead of HardRock if you're using it for speed, but Speed Rate can also override HardRock and add more difficulty and your choice of speed.

Sudden
Just like Hidden, the notes will be invisible for a period of time. Except this time, it's alternated. The notes will now appear very close to the button area, which also makes it very hard to know which notes are coming up next (unless you've memorized it).

Mirror
These will mirror the notes. Ex: A note that was all the way on the left will now be placed on the right.

Add Mines
A feature used in most of the DDR series, these will decrease your health by a LOT if you hit them. These can also be added by yourself in the edit menu. The skin for these will not be huge, as they will be like a regular note, but flashing red and black.

Add Notes
Certain notes can be added into the game by yourself, without going into edit mode. Certain modes like Stream, will add a extra note to a area with just 1 note. Quick, can add 1/4th notes and 1/8th notes. Others you'll have to find out for yourself.

DDR-style mod menu
This can be added in the settings, or (if ppy wants to) by simply clicking the More Options button when you start a song. It'll have a count down of 3 before the song starts, just in case you really want to. This is only, and I mean ONLY in osu!mania.

Well, this is all I could come up with. Let me know what you think.

sHaDoWsN1pEr13 wrote:

Speed Rate (2.0x, 3.0x, C250, .etc)
These increase the speed of how the fast the notes come at you. I personally would use this instead of HardRock if you're using it for speed, but Speed Rate can also override HardRock and add more difficulty and your choice of speed.

all ready added. press F3/F4 to change the approach speed

sHaDoWsN1pEr13 wrote:

Sudden
Just like Hidden, the notes will be invisible for a period of time. Except this time, it's alternated. The notes will now appear very close to the button area, which also makes it very hard to know which notes are coming up next (unless you've memorized it).

the Fade-in mod all ready does this

sHaDoWsN1pEr13 wrote:

Mirror
These will mirror the notes. Ex: A note that was all the way on the left will now be placed on the right.

don't see a need imo

sHaDoWsN1pEr13 wrote:

Add Mines
A feature used in most of the DDR series, these will decrease your health by a LOT if you hit them. These can also be added by yourself in the edit menu. The skin for these will not be huge, as they will be like a regular note, but flashing red and black.

please no :<

sHaDoWsN1pEr13 wrote:

Add Notes
Certain notes can be added into the game by yourself, without going into edit mode. Certain modes like Stream, will add a extra note to a area with just 1 note. Quick, can add 1/4th notes and 1/8th notes. Others you'll have to find out for yourself.

why D:

sHaDoWsN1pEr13 wrote:

DDR-style mod menu
This can be added in the settings, or (if ppy wants to) by simply clicking the More Options button when you start a song. It'll have a count down of 3 before the song starts, just in case you really want to. This is only, and I mean ONLY in osu!mania.

wait, is this for changing the song selection screen?

Speed rate: Already there. Use F3/F4, as deadbeat said.
Sudden: It already exists, and it's called Fade-In.
Mirror: Don't see a reason for this, as it likely doesn't make the song any easier or harder. But for the time being, you can flip songs through the editor (but if you do this, it'll make them unranked).
Add Mines: This was part of the Hidden mod before they changed it, and removed them due to the response from the community.
Add Notes: I don't see why this is necessary, or how it could be added without messing up some songs' rhythms.
DDR-style mod menu: There already is a mod menu though. Why bother changing it just to imitate another game?

Argument for Mirror.
Some players press the center column (in 5K or 7K) with a different hand (in my case, left, while most people do right (even though I'm right-handed)). Mirroring can help some o!m charts "designed for right-handers" for left-handers.
(Designed as in the sense like 3 left columns for something and 4 right columns for another thing which suits right-hand players; by mirroring, we have 4 left columns for something and 3 right columns for another thing which suits left-hand players.)

I'd like to see Add Mines, but I doubt the same response will be given by others. I don't want Add Notes though; seems like it throws the rhythm part away.

chaotic_iak wrote:

Argument for Mirror.
Some players press the center column (in 5K or 7K) with a different hand (in my case, left, while most people do right (even though I'm right-handed)). Mirroring can help some o!m charts "designed for right-handers" for left-handers.
(Designed as in the sense like 3 left columns for something and 4 right columns for another thing which suits right-hand players; by mirroring, we have 4 left columns for something and 3 right columns for another thing which suits left-hand players.)

I'd like to see Add Mines, but I doubt the same response will be given by others. I don't want Add Notes though; seems like it throws the rhythm part away.

Excuse me, DDR is a 4k game.

But osu!mania is 4-8K game. If the Mirror mod is only available for 4K, I'm against it to all extent. (This applies to all mods that are only available to certain number of keys.)

These mods are all pretty bad ideas (excluding the ones we already have of course) if anything I'd like to see DJMax Chaos style mods.

Chaos Wave
http://www.youtube.com/watch?v=yrpPSd7vUgw

Chaos X/Y (makes all lanes have different speeds)
http://youtu.be/xDhoc2UxD0Q

Can't find a video of Chaos Slide-Up/Down.

and.........
boom