Other Worlds

[Cynor]

Awhile ago I made a post similar to this one and got some pretty good responses to it. However it made me realize that I maybe needed to both be less specific but more strict on what I wanted. Which I know sounds weird so please just bear with me as I explain what I mean...

The original thread was basically "trapped in a game world". Think .hack and Sword Art for prime examples. That thread, along with some new series I've watched, have made me realize that maybe something dealing with other worlds would be better and open up more choices.

Note: For this I'm defining a second world as anything not earth. So something like traveling between multiple planets in space could count. I will also count things like the spirit world/afterlife as not earth, but this isn't as ideal.

However I still want to keep a certain theme here, so it needs to meet a couple requirements...

1. There must be TWO worlds involved somehow, one of which must be OURS.
2. This second world needs to be fairly clearly defined and an actual world, not just some vague thing like "here's the aliens". It can start off as a little mystery, and maybe the true nature/purpose is still mysterious towards the end, but it can't be a "oh we're getting invaded and know nothing" type of thing.
3. In addition it needs to figure predominantly in the story. Be a major plot point. It shouldn't just be mentioned and tossed aside, only mentioned/seen in a couple episodes, etc.
4. While the second world needs to be central to things, neither world can be tossed aside for favor of the other. They don't need to be treated equally, but both need to drive each other in major ways.

Some examples of this I've seen/remember:

Sword Art Online, .hack,and Log Horizons - "Trapped in a game" style, and in all cases the real world still plays a huge role since they want to get back to it.
World Trigger - The opposite of the above series since it takes place on earth, but the second world is constantly there and drives things forward.

Possibly good examples (either not seen or don't quite remember):

Accel World - Not seen, however it seems like the game really drives things and is central, but there are real world ramifications/issues for the characters too.
Serial Experiments Lain - Been awhile since I've seen this one, but iirc it could fit since she is exploring/being effect by both the real and cyber world.

Not so great examples:

Angel Beats - Great series, but the real world is hardly seen at all here. Just a few flashbacks at end.
Clannad After Story - Another great series, and ALMOST makes the list as an ok example, but while the second world is important, I don't remember it being explored much.
No Game No Life - Another "almost ok example", since I believe that they want to get back home when done playing games, but at the same time I believe the earth is just kind of forgotten. The new world is much better for them so the old goes poof.
Shuffle! - Similar to Clannad. Oh hi, gate to another world! But we're mainly here to bring in different characters than humans. Don't remember spending a lot of time in that second world...

[SeijiSensei]

Noein has travel between this world and two parallel universes.

[kitten320]

Fairy Tail had an arc like that, but only 1 arc

Hunter x Hunter had a game arc

Arata Kanagatari
Dog Days
Inuyasha

[AntonKutovoi]

This season's Overlord have a protagonist trapeed into another world. But so far he didn't try to find a way back. In this sason there's also Gate - JSDF against dragons.

Other examples - 12 Kingdoms, Magic Knight Rayearth, Harukanaru Toki no Naka de, Kekkai Sensen.

Oh, and there's also Sonic X, which also fits your description.

[Tenzen12]

Birdy the Mighty Decode
Hagure Yuusha
12 kingdoms
Ixion Saga DT
Outbreak company
Unhibtated planet Survive

[frodonk]

Hataraku Maou-sama!