1. Earth gets nuked, causing a severe change in it's appearance. (adventerror has the earth change color from radiation, adventure time has the earth have a chunk blown up from the blast)

2. Weird creatures appear on Earth, including previously non-sentient things turning sentient. (the non-sentient things turning sentient being the stickmen from adventerror and the animal and food people from adventure time)

3. There is a severely low amount of humans left on earth. (adventerror has 8, while adventure time only has 1)

4. Finn is similar to Tera, as they both share a similar role, and are also involved with god(s), with Tera being involved with Corruptori, and Finn being involved with Glob.

5. There is a catastrophic event near the end involving an evil god. (rifa summons zenkami bringing the world into despair, and king man, betty and maja summon golb bringing the world into despair)

(thanks to Nayerb for telling me the first one)