Firstly, saying only +support or arguing over another rhythm game that has nothing to do with Touhosu! is a good way to get this thread locked.
As many people have said, the trick to this mode is keeping both the rhythm game feel of osu! and the insanity of bullet hells. Here is my suggestion: There needs to be the ability to shoot bullets as otherwise people only focus on dodging which has less rhythm and shooting allows you to get score. You can have the character automatically fire (or if you hold fire button) at a rhythm determined by the beatmap. There can be lots of enemies that pop up like fairies and spirits popping up according to the position of the notes in the original beatmap, but can come from edges of the screen too. Having enemies would make sure that people focus on something other than their character, and not just a boss in the center that you have to stay under to get max points. Personally I don't like the idea of grazing giving score (I don't like this in Touhou either >_>) and it makes more sense to have a combo gained by killing enemies. This would be more like a standard touhou stage or standard stage combined with Danmakufu because there would be lots of bullets plus enemies. Also it would be cool to have an 'All Clear' thing on beatmaps after you 1cc them.
Another difficulty brought up is making sure that the autoconverter would be able interesting patterns-- Sakuya Lv9 even suggests not implementing an autoconverter at all. I disagree with Sakuya, as the whole point of extra modes is being able to play all the songs in a different mode. Peppy might as well make an entirely different game if you couldn't use the already created beatmaps. Of course, Touhosu! specific beatmaps would be much better for this mode like other mode specific beatmaps are better for their respective categories.
An example of something sort of like this could be the first boss in 140:
http://www.youtube.com/watch?v=aN_aTIWQ81sYes this game is really easy and it shoots too slow. Also saying that an idea is bad because it's too easy doesn't make sense because peppy can adjust this very easily by making it have more bullets & there will be multiple difficulty settings for maps anyway.