If you have played Super Meat Boy, you will know of things called Warp zones and Dark world. If you haven't played it, the warp zones are portals which take you to a set of levels based for one character normally a unlock able one, or they are harder levels with old school game art.
Dark worlds are based on the normal levels, Dark world puts a spin on this levels, this normally makes the harder to complete.
The Nether gate im adding will work the same way as this but, it will be a select few levels with the nether gates in them, theses levels will be a bit harder than the original ones.
The nether levels will feel like the original ones but it will look like the nether.
I have also started to try and to some npc (pigs, cows, ect) In the video you will see the pig moon walking, this was just a test to see how I can add them, Im thinking of a couple way of adding them.
1. Make them follow the play but they will not be able to jump.
2. Make them just go left and right not following player.
Tell me what you think will be the best, Im also thinking of how to do creepers as this would mess the level design up e.g. Blowing holes in the floor, breaking levels, impossible to finish.
I will add the other mobs, and add the "power" which should be easy (I think)
The video contains what I just talked about above.
Thanks and Have a good day :D