The Visitors

Or they could be more logical and have there be invasion after failed invasion. This would also make sense with the crystonal time extensions. Miss it this month? maker sure to extend your equip in the chance to get it next time!

Also..... maybe the weapons and armors and such progress each time you complete a cycle. So say you have the 37 weapon, and next time around, it takes that and gives you the next tier weapon.... it would completely make other weapons and such WORTHLESS!!!!

that's less logical because it's supposed to be a continuous story with a beginning and an end. you should only be able to defeat dr. bing once, but if each wave is lead by him that wouldn't make sense. with this being heavily influenced by time travel, a paradox is a very real possibility, allowing time to repeat infinitely and dr. bing to be defeated continuously for all of time.