Hello, this is my suggestion. I hope this thread isn't a duplicate one 
(If so, immediately give me a link which directs to the thread and lock this thread)
How about add a killing object? I mean, when we hit the object, you lose some health and cause combo break. But, it will be different "how it works" on different modes. Sounds like more challenging to play. Here are the explanations for each modes (Update: including illustrations (Warning: may kill your bandwidth!)):
And when we use SD mod, you will suddenly fail. But in PF, when we hit the killing object, you must start from the beginning because you hit the wrong object.
I hope i don't lack of details here
. If you feel so, just ask me a question and i will answer as good as i can. Sorry for my english if you see any wrong grammars 
. Hope everything's clear with those illustrations
(If so, immediately give me a link which directs to the thread and lock this thread)How about add a killing object? I mean, when we hit the object, you lose some health and cause combo break. But, it will be different "how it works" on different modes. Sounds like more challenging to play. Here are the explanations for each modes (Update: including illustrations (Warning: may kill your bandwidth!)):
osu!standardWhen we click the killing object, the health is reduced and cause combo break. The closer with the hit object, the harder the difficulty to finish them the harder to click the hit object (Maybe a spiky silver hit object?)
Description: When the hit object appears, the killing object also appears at the same time. The Approach circle approaches the object as usual, only for hit objects, not for killing objects. When the hit object has been clicked, the killing object disappears.
TaikoQuite the same as osu!standard but the difference is, by hitting the killing object, you lose some health and cause combo break too (Maybe a silver or gray taiko hit object?)
Description: When the blue and red hit object approaches "hit area (?)", sometimes, the killing object (gray one) approaches the "hit area (?)" faster or slower than any hit objects. To avoid them, do not hit the gray object.
Catch the BeatStill the same, but only happens when we catch the killing object (Maybe the object will be durian fruit?)
Description: The killing object falls down along with the fruits. Sometimes faster, sometimes slower than the fruit. To avoid them, do not catch them.
osu!maniaSomewhat the same as osu!standard. When we hit the killing key, you lose some health and cause combo break. (Maybe red coloured key?)
Description: Notice the red key. It's clearly different with other keys. The red key approaches along with other keys (Just like mines in stepmania)
And when we use SD mod, you will suddenly fail. But in PF, when we hit the killing object, you must start from the beginning because you hit the wrong object.
I hope i don't lack of details here
. If you feel so, just ask me a question and i will answer as good as i can. Sorry for my english if you see any wrong grammars . Hope everything's clear with those illustrations 
.
. But if you want to set them as mod, then it will add more killing objects than one (not exactly like a hardrock, where it only increase difficulties)