Is there a mod which does to Ayleid ruins what Snus dungeons does for caves?

Not really, no. There are a few mods that expand isolated ruins - CDEP Fanaceya (Fanacasecul, Haynote Cave, Ceyatatar), and NNW's Ayleid Ruins Expanded has a questline that involves several existing ruins (some with new areas/levels) and several new ones. Scot (the guy who did Better Dungeons) had planned to an Ayleid expansion, but life got in the way and he hasn't been able to work on it.