Let´s say, you had the opportunity to make your perfect game, what would it be like? Shooter, RPG, Adventure? Horror, fantasy, sci-fi? Any special mechanics or unique ideas?

It'd be like an RPG most likely and it'd have a pseudo fantasy setting, gigolo mode, boss rush mode, and a soldier drop kick.

A sci-fi flight simulator along the lines of
http://www.mobygames.com/game/g-police
and
http://www.mobygames.com/game/windows/hardwar
and
http://www.mobygames.com/game/street-rod
with no DLC of any kind.
edit : you should have your own hanger/apartment where you can completely dismantle your craft and install new components. I want to see threads created by players who managed to completely strip their craft without taking notes, and are left clueless as to how to reassemble it.

sigh...

Why would I remake Deus Ex?

Something like Postal or Carmageddon only without all the reading and strategy and a lot more senseless violence.

If you change your question to : "If you had the opportunity to force any developer by gunpoint to develop a game, what would it be?"
then either:

Warcraft 4 (PC) ,
Mech Commander 3 ,
Outcast 2 ,
isometric Star Wars RPG ,
or a decent Age of Empires sequel.
Actually scratch the last one : I want a decent Empire Earth sequel, with the same vibe as the first one.

6x sigh...

But if you have just one chance, to make your personal perfect game...

why scratch age of empires? i personally loved aoe2 and lost hours upon hours to it.

its would be an action-rpg-adventure hybrid with open world housing total freedom as to characters creation optimazation with a sandbox like crafting system in a scifi-fantasy setting :) this one exists its called Marvel Heroes :P

Hmm maybe Jade Empire but with Soul Calibre standard combat; Fable style melee, ranged and magic switching, Skyrim's scope and openness but its set in a Deus Ex style cyber future with hacking subgames and multiple ways to approach a level and GTA style driving/racing missions only the racing is better than GTA - as good its as it is in Flatout: Ultimate Carnage and aircraft combat like Crimson Skies or G*Police
THERE!
And I want it on my desk Yesterday!

I like games of different genres.

Putting the features together into one game would ruin it, IMHO.

Shooters with good story, realism and tactics like Arma2 with ACE mods (hate those jumping around and fast shooters like CS, Quake etc. since I became older).
RPG with good graphics, books and stories (important to be able to adjust the setting to your liking through mods or editor-tools).
Games like OSMOS (I spent easy 100+ hours on this :D)

Sometimes, a game looks bad at first but then becomes one of the best you played. I notice this with Two Worlds Two at the moment, it was very hard to adjust to the movement and then it just becomes second nature. No need to concentrate so much on movement, you start enjoying other things in the game.
This can be very basic stuff. For example in Risen1, the S key moves the character backwards but you keep facing the enemy like a retreat, in Risen2 the character turns around and shows the back to the enemy which results in lots of damage. Those things are really annoying at the beginning until your adjusted to it.
Especially RPG do not show their full potential at the beginning.

Bad stuff in games are artificial time-sinks.

Far as I know Marvel Heroes only lets you play Marvel heroes, which is not what I want. That's unlike DCUO for example.

I have MH 2 and while it is only Marvel, it is a very extended list.

Some ideas:
Hitman meets GTA
Civilization meets Panzer General
Civilization meets Industry Giant
Far Cry meets a dating simulator: Once not paid attention and being in reach of the arms of a freak/monkey trigen, there is no escape