Front Mission + Valkyria Chronicles = Semi turn based tactical squad combat featuring giant custom "Wanzers" armed with over sized rifles, machine guns, shotguns, missile launchers, mortars, hand to hand weapons and huge riot shields. Valkyria Chronicles has an excellent combat system and story, but suffered from a WW2 in Europe knock off setting (personally I think a lot of people were burned out on this because of all the early Medal of Honor and Call of Duty titles that had come out recently). Front Mission on the other hand has flat characters, but a cool setting featuring a politically chaotic near future world complete with rogue nations, PMCs and terrorists. Combine the best of both of these and you'd have the most awesome robot combat game since Mechwarrior.
Master of Orion + Mass Effect = Imagine a space 4X game featuring exploration, colonization, diplomacy, espionage, technology, space warfare and planetary conquest set to one of the most popular and richly detailed sci-fi backdrops invented in recent years. Choosing your starting faction? feel like playing a short game? Go Terran. Want a long game? Play Asari. Looking for a fight? Choose the Turians. Need a challenge? Try the Krogan. No shortage of art assets to use. Plus a procedurally generated universe with random elements would give the game a lot of replayablility. Think of the events of the Mass Effect series as if they were only one of many potential events in a much longer sequence. Of course the end game could always go out with a bang via Reaper invasion.
Ace of Combat + Wing Commander = The peanut butter and jelly of video games. In their day flight sims where the pinnacle of PC gaming, boasting the most impressive graphics and immersive gameplay. Sadly those days are long past, but there's no reason why a revival can't give younger audiences a glimpse of this genre's former glory. Ace of Combat does a good job of capturing the intensity of dogfighting and also injects a fair amount of cinematic flare. The problem is the setting. It feels like the worst of both worlds in that it's neither the real world we live in nor is it fantastical enough to give the creators artistic freedom. Wing Commander on the other hand has a great setting which has been criminally under represented . Instead of having a few fighters dancing about an empty nav point why not a nebula? Or the rings of a gas giant? Or near a pulsar? How about over the surface of a white dwarf star? The tail of a comet? What about in the corona of a red giant slowly being devoured by a black hole? Time to let the players fly for the Kilrathi too.
ULTIMATE FEUDAL SIMULATION.
So, those are just a couple of fusion ideas I had off the top of my head. If you readers have some ideas of your own don't hesitate to post them in the comments. I know a lot of what I suggested above sounds pretty outlandish, but I've grown tired of seeing the same couple formulas involving a FPS/MMO/RPG combo or some generic competitive/cooperative multiplayer attachment that hardly anyone is still playing a week after launch. It's getting bland. Time for some new recipes.