Hey there 
I have an idea of a game and would appreciate some advice. Let me describe, with an example, the basic structure of the idea: five people playing a level in a multiplayer mode (nothing unusual there) but one of them is different, he can monitor what other players are doing and he can also change the world in real time (adding/removing objects, for example). Is this scenario plausible? If so, to what extent?
Yes, it is plausible. One particular player would just have abilities and views that the others don’t have.
If I recall Vampire:The Masquerade did something similar to that, with a Dungeon Master like character controlling the scene as 4 other played through the level regularly.
It’s certainly doable, and will just require some extra work as you will be building a mini editor within your gameplay for the Master to work with.
Thanks for the (really fast) replies! 
Gaah!! A game master is EXACTLY what my project’s all about! I didn’t use this term because I realize very few people are familiar with this concept, when I say RPG they immediately think computer RPG and I have to explain the whole concept of tabletop RPG and game master.
I’m glad this is possible with the Unity engine but also a little disappointed, because I was certain this was somewhat an original idea… lol I mean, it’s plain tabletop RPG but for some reason the game master concept never made it’s way to online computer games.
Anyway, thanks again I know people are probably tired of “can Unity engine make X game?” kind of thread.
visual game masters like this actually existed for a long time.
be it in non-interactive forms where the GM uses it for a real p&p form where its just 2d for him to keep track of all or through such in game methods.
none the less it is that rarely used that it is a fresh concept even if not all new
so a lot to do in an innovative way