On seasonal we have a chance to get an ancient weapon invasion that drops Tuvala mats, I think the event is a nice way to break up the grind.  

Can the same type of event happen on normal servers as well. Instead of Tuvala mats, have the mobs drop Ancient Spirit Dust, black stones, and whole Caphras at whatever rate seems balanced.  Then have the Layten have the potential to drop Tungrad accessories, Layten Neck, and the 25-50 caphras bundle.  If you really wanted to go crazy, give the Layten a chance to drop compass and Map pieces.

Edit:  As mentioned below, This could be added to all grind spots.  We already have Nagas that invade the Fogans in Elvia, but we could have Castle Ruins, Bloody Monestary, and Biraghi den invade each other.  Gyphin mobs could invade everywhere in Kamasylvia and O'delitta.  The list of ideas goes on.  

This would be great. I'd even recommend extending it beyond just Ancient Weapons to having 'invasion chances' in each region with fiends endemic to that area. Maybe a pack of goblins suddenly appearing you have to fight off just outside velia? Or a horde of Orcs or Biraghi bandits in Serendia. Image an invasion force of giants suddenly appearing near Keplan. Maybe an invasion with the monarch of darkness with minions in toe. Or Elric Shrine cultists in Mediah. The options really are endless, and like you say - what we have at the moment is enjoyable on seasonal servers when it happens, no reason that positive addition can't be expanded on. 

There are so many great things that could be done with this mechanic that they already have in game, great suggestion.