It would be useful if ikiwiki could reject adding file with names longer than some limit, like 255 bytes. I've had some git-annex users complain that some files ended up longer than that, which won't work on some OS's and filesystems.
255 might be a good default value, even.
This would apply to pages created following broken wikilinks and using blogpost forms, as well as to uploaded attachments, and perhaps aggregated pages. In the wikilink case, I guess ikiwiki would need to reject it as too long; in the other cases it could simply pick a shorter version of the proposed filename.