@Background Pony #A488  
I always thought of Sunset being Liquid, but then she just turns into Raiden afterwards.

My version of the Metal Gear Solid cast. (In case anyone actually cares)
 
Twilight Sparkle=Solid Snake “A surveillance camera?”
 
Shining Armor=Colonel “I need scissors 61!”
 
Firefly=Miller “Come on, start talking byayh!”
 
Flash Sentry=Meryl (It’s more of a close friendship thing than actual family relations) “I’m no rookie!”
 
Trixie=Revolver Ocelot “Trixie must admit, you’re pretty good!”
 
Moondancer=Cyborg Ninja “Come Twilight! Make me feel alive again!”
 
Spike=Otacon “It’s like one of my comics!”
 
Pinkie Pie=Psycho Mantis “You seem to like ‘My Little Pony’, don’t you?”
 
Rarity=Sniper Wolf “I never give up a hunt, darling. Now you and your tacky suit are mine.”
 
Big Mac= Vulcan Raven “Eeyup, the stars shall mourn the loss of a child tonight.”
 
Sunset Shimmer=Liquid Snake “Not yet, Twilight! It’s not over yet!”
 
Starlight Glimmer=Solidus Snake

Well, Celestia is absolutely The Boss. Who else could possibly fill the shoes of Badass Mother Figure? I always thought Luna would be Big Boss, then Twilight would be Solid Snake, personally.
 
Spike could be Johnny Sasaki, lol.

In my opinion, trying to exactly match up characters isn’t very interesting as far as crossovers go. You usually end up either trying to shoehorn characters into roles that don’t really fit them, or extrapolating them to illogical extremes, which means losing what makes one or both of the characters who they are. I find it better to try and fit Character X into the world of Series Y.

@Da5ch  
Or Psycho Mantis.  
“You like Koneighmi games, don’t you?”
 
Also, Spike may work as Grey Fox. And, needless to say, Twilight’s mother has to be Big Boss, with either Princess Celestia or G1 Twilight as the Boss before her.

@FanOfMostEverything  
EXACTLY. But poor Spike needs a better role than the first thing that came to mind for me just now, which was the little Metal Gear Mk. II robot from MGS4. That’s kind of a pitiful role for a character as important as Spike.
 
And now I’m wondering what Revolver Discord would look like with one of his limbs replaced with one of Liquid Trixie’s front legs. And for that matter who would be Grey Fox? I was thinking Pinkamena, but then I realized Pinkie would probably fit better as Laughing Octopus.

@Da5ch  
And really, if any FiM character’s going to graft dead limbs onto his own body, it’s going to be Discord.

@FanOfMostEverything  
Oops, that was actually me…I forgot to log in again, so it says my reply was “Background Pony.” Anyways, my friend and I thought to make Discord be Revolver Ocelot, just because in our opinion he’s the most iconic MLP villain, and Ocelot is easily the “big bad” of Metal Gear Solid.

@FanOfMostEverything  
You are completely right about Solid Sparkle and Liquid Trixie. In fact, the main reason why I decided to draw Solidus Shimmer was because the fandom seems to mostly cast Twilight as Snake (obviously because of “It’s About Time,”) both Solid Snake and Big Boss/Naked Snake, and Trixie as Liquid Snake. So why not cast Twilight’s OTHER arch-rival unicorn as the third and final clone of Big Boss, Solidus Snake? Also I chose her for that role because the names “Solidus Snake” and “Sunset Shimmer” both have the initials “S.S.”
 
Now as for Spike, I unfortunately have no idea what kind of role he should have, since me and my friend agreed that Fluttershy would fit best as Otacon (even though that would involve her pissing everywhere with fear, probably embarrassing her, lol,) whereas Rainbow Dash is Raiden, or Raiden Dash. I mean there is a fair amount of fanart on this site that casts Dashie as Raiden, especially Revengeance-era Raiden.

Hmm. Presumably, this makes Twilight Solid Snake and Trixie Liquid. Not sure if Spike should be Otacon or Raiden. And who is Revolver Ocelot, Snips or Snails?