I think this is an awesome idea; I can't tell you how many times I've gone to Bemanistyle and looked for simfiles of songs I found on osu! that I liked and realized there are none, or how disappointed I am by the lack of DDR/ITG song beatmaps here. So something like this would be a good medium even if it doesn't have to be as fleshed out as SM.
Just a few things for discussion though:
-Scoring: One of the big parts of osu! is keeping a combo, that's how you get the most points, and if you miss one somewhere in the middle you get a significantly lower amount of points. In ITG a miss will break your combo but you can still score well, so will scoring in this special mode be closer to osu! or closer to ITG/DDR?
-Timing windows: ITG has...6 timing windows? osu! basically has 4. And other rhythm games have differing amounts too. If this mode were implemented I'd assume it'd be perfect (300)/great (100)/good (50)/miss but maybe not.
-Arrows: Will the notes for this be the circular beats that normal osu! uses or will they be arrows or the beat things like in O2Jam/IIDX/GF/DM? Also, in DDR/ITG the arrows are colored differently based on if they're 1/4 or 1/8 or 1/16 notes, etc., would this mode just have all monocolor notes or one color for each column (similar to Pump It Up) or what?
-Randomness of charts: One of the worst things about mods in DDR/ITG is how ridiculous they can get, especially if you play on Random/Blender (or whatever they're called, I don't use them so I might have the wrong terms). I hope there will be an algorithm or something in place to prevent stupid shit from being generated in the chart, because it sucks (and sometimes is practically impossible) if you're playing on blender and you get something like this:
I dunno if anything I said has already been addressed because I read the thread last night but didn't reply and can't remember all of what was in it. :x