I think it changes vastly from person to person. For example, I always have a lot of different ideas that can be turned into stories ; sure they have similarities with existing works, but they're all independent and 'original'. My own problem is turning those ideas and stories into a coherent...
Well, first BL and GL are just a special type of romance. And romance is not the same as smut : you can write a perfectly good love story without any sexual scenes or even undertones. However you can of course easily combine both.
For Harem it can be either multiples love interests, in witch...
It almost always involve magic. But form a more general point of view, you can say that it needs to be a world that involve different laws (physical laws not human ones) and functioning than ours : it can be magic, monsters, ki...
The important point is, if the physical laws are exactly the same...
For me, I would definitely read the first one, it can be really great if you succeed in representing goblin society and the trouble of mc adapting.
The second one could be interesting too, but it seems less interesting.
The other three are not my cup of tee, while the third COULD be but I...