One of the problems isn't so much the engine. Vegastrike, Fightgears and so on are really good engines. The problem is GFX and SFX. Making those high resolution textures tends to require good artists many of whom are going to want some form of remuneration for their hard work, same with SFX, using off the shelf noises and music might well incur licensing issues and such.
Of course, if you can get the team together there is nothing stopping you making the next 'Dragon Age' or such.