Well Rossi, u need huge money, knowledge and a very good crew to realize it.
At first u need a very concrete idea, 'cause all bases on that. What kind should the sim be? More realistic as all the others? So u have to look out for guys who knows more 'bout the planes and all the other stuff that moves around. U need very exactly datas 'bout the planes, the tanks, the missiles and so on.
Than u have to find good modeler. They are very expensiv man, hell i know that, thinking off what my customer pay for my models ;)
Huge maps to model with thousands of objects, the sky, the water.
Physics like pieces bumping on the ground on a hit or explosion.
Biggest and most difficulty thing is imho the AFM, most of the sims doesnt have a really good afm.
Hell, there are so many things to think on that i get a headache ;)
But if u really wanna make it real, go on! No matter which stones on your way, what the others say or how difficult it would be....just do it!
No try, no success...