User blog comment:TheIrregularOne/Why not OVAs?/@comment-11437688-20140106002621/@comment-7263151-20140112021315

I'm pretty sure Negima was redone with a whole lot of OVAs... so I think it happens in certain cases if the original anime isn't that good. But I think it's rare.

Also as far as anime viewers are concerned, the side-stories are original stories, so it would work if they were done in OVA's.