The natural regen doesn't make too much a difference for union events. The less you play, the less you'll use. The more you play, the more you'll use and it's not like the seed count drop rate increases the longer you play so both should have the same use/gain ratio. Union events experts only cost 1 BP, so you can run it nearly forever as long as you just do experts.

Raid events on the other hand, since the new soul drop system, has been hurting my seeds because Ragnarok cost 4 to run while possibly providing no return in seeds.

To answer the topic, we 'should' be having an illusion element union event. Emphasis on 'should'. With several other union and raid events coming up one after the other. We're not 100% sure yet since it seems they haven't updated the side banners, maybe they caught on that people figured it out, not that I see any harm in it and nutaku has not followed the dmm event order before. We also don't know what they plan to do with the reprint events the already gave us early, which if they don't fill in with anything, we're going to have several consecutive union and raid events.