I don't mind the questions. It's important to know if you think alike or it's better not to start at all. 

I like the real world and fantasy mix. In Charmed there were many places that don't exist in the real world, like the underworld, the amazon island Leo gets banished to by Chris, the heavenly place where the elders live, the demonic market, the astral plane. In Harry Potter there are regions which are unplottable. Muggles can't find them or draw maps of them but the magical community can.

So basically you could create a magical world within the real world or go to another dimension/plane. Alternate universe is possible too, either as a 'we got transported here and need to find our way home' (Wizard of Oz/Alice in Wonderland) or as a 'we need something and can only find it here' kind of plot. 

I was playing around with the idea of a bar where only magical creatures could enter. You say 'magic' to the doorman and if he sees the magic in you he'll let you in. Both good and evil are allowed, fighting gets you kicked, but the barman there has a lot of knowledge, he's like a walking library but his info costs money (or magical currency). It's not the right setting for an adventure (I meant it as a leisure thing) but it could be a place to start the adventure or return to when the characters are stuck if you like the idea? 

As for the amount of people involved I think a party of 3-5 and a GM (or a GM who also plays a character) would be fun. 6 people is already a lot if you want to keep going. For example: it's 11pm here so bedtime.