Ok, I'm rather shitty at quoting post parts, but I'll give you my responses to what you said.
Firstly, thanks for taking all of what i said the way i meant it, honest criticism meant to help.
I agree that pendants aren't too creative, and the sliding thing has been done, but those were only quick pitch idea's.
The main concept I thought felt strong was the idea of his theft being the action that ports him, and it possibly being linked with his girl.
The first thing I will say overall with the info you gave me is what I'm by far the most sure of, so hear me out on this.
The concepts of;
best friend betrayal
God and devil powers
becoming a demon(biggest one, i would personally cut that entirely)
are very very done, and its hard to not make them lame or boring.
I mulled over the general feel and progression of your plot and came up with this
Take and leave what you please, and tell we what you think of it.
I'm also going to be generalizing things, so that you can add your own creative touch to this.
A theif steals some sort of object, or activates some sort of device that teleports him into the middle of a murder scene. Disorented and confused, he is quickly aprehended by the law force of the area. He is easily convicted and sentenced to death(perhaps a language barrier causes this, or he just ended up very close to the murder weapon, w.e).
In steps the head figure of this culture, town, or civization. He offers to save the theif, and even help send him home, Granted he follow his instructions. Here you can send him on those missions of small significance. (steal some random thing, redict soem mail, w.e and then do the concept you had of small things having wide and bad effects.)
He starts to get really angry with this guy for tricking him in these ways,speaks out and the, we'll call him "mayor", traps him in a ruin (think aladin getting eaten by the cave of wonders type deal)
While hes trying to find a way out, you could maybe introduce a type of destined figures into the story using some wall paintings or w.e
I would really shy away from the god and devil angle, but also I think you should really try and shake it up here.
Really mess with the concept of a destined hero(the theif) and a destined villian(mayor guy)
I don't want the theif to be super just and good, I wanna see the player unsure who the bad guy truly is. All the theif wants is to go home, and he has to topple the mayor to do so. Don't reveal who is who till its dramatically best to do so.
Also, If you want my recomendation on how to do a new type of "destined hero and villian" how bout this?
One figure is the embodiment of the people's selfishness and xenophobia, the other is an embodiment of the people's desire to see power wielded to change the status quo.
This allows you to really mess with the player, not letting him figure out which figure is the good guy, or even which figure the theif is. Hell, you don't even have to tell them.
You can then have him return in some way to face off against this mayor person.
Thats the general progression I think allows for you to have a compelling, not-too-run of the mill plot, that is easily controlled when it comes to gameplay length(the sections of "Intro, Missions, Cave, Faceoff" can be either very short or very long as the concepts are quite flexable). It also allows for alot of creativity.(The place he warped to's culture and style, the mystery of the theif and "mayor"s roles, maybe soem ancient clues or artifacts in the ruin he explores, the mayor himself, the ending and true natures of the characters) There are alot of options I think. I personally am invisioning the place he hails from feeling slighty westernized Edo, and the place he warps to a sort of cyberpunk Arabia. With a sort of Visor overlord. The idea of a cyberpunk Arabia can be really streched and ran with, because the simplisity technology wise of the Arabian setting can effectively let you pick and chose what tech you want present, and I personally think the clashing of the two sets a very cool stage for a mystery adventure.