For osu!mania, I feel that rankings should be separated. There should be 1 ranking for 4K mode, another for 5K, and so on.
The score penalty for having a mode on that isn't the preset one is really unnecessary, and having separate rankings for each game mode would more accurately represent each player's skill in different modes.
I'm not really sure how this would work for o!m maps.
And as a change to having the different key modes in the mods menu, perhaps a key# selection screen either before the song selection or a screen after song selection would be nice.
The score penalty for having a mode on that isn't the preset one is really unnecessary, and having separate rankings for each game mode would more accurately represent each player's skill in different modes.
I'm not really sure how this would work for o!m maps.
And as a change to having the different key modes in the mods menu, perhaps a key# selection screen either before the song selection or a screen after song selection would be nice.