I can confirm that the next collection unit is indeed Metus (yay!) but she's far along the time line of events and that's assuming we don't get other collection events that Nutaku skipped. I believe we have yet to get Liana's event for some reason.

On the issue of revivals, I can't really say much. Going by the japanese wiki it seems that revival events are somewhat sporadic at best and they seem to revive mostly the more popular/older events more frequently. A good example is the amount of revivals both Karma and Anya have received by now where as Eterna only had one revival back in April.

Thankfully Nutaku players have the benefit of "looking ahead" and see if the next event is worth the investment. Ultimately the decision of to perfect or to not perfect depends on the player.