It would be clientside. The arcade API has support for simulating the same thing the original client sees by playing back the input the original client put in. The timing would be correct, but delayed.
I don’t think we will be making a DDR clone, though. Konami has strict patents on the genre. Pretty much every rhythm game that has notes falling from the top to the bottom has to pay a royalty to Konami. I wouldn’t rule it out as something someone can make for Workshop, though.
Pinball would definitely be possible, but we’d have to create our own custom physics for it. We’d most likely not do this for a long time, though (if we did decide on making it; it’s practically its own game).
We’ve talked about Gachapon’s internally before.