My two cents worth...
I went through the last posted list and am suggesting some removals from the list with the reason being listed after. Only a few are for an opinion based reason, most are relative scarcity. Just throwing it out there. if they say 'hard to obtain?' that means I am unsure of their commonality but think they may be hard to get. Please correct if they are easy to get!
American Ninja Hard to obtain?
Big Trouble in Little China (While I love this movie and would love to see it riffed it has 2 problems in my eyes. Kind of hard to obtain and is already a comedy/satire might be hard to do.)
Blair Witch Perhaps not Mike's cup of tea. Chock full of F bombs and not much else.
Cabin Fever Not really up Mike's alley. Really gory (not that I mind that, but for normalized consumption)
Dick Tracy maybe hard to obtain. Not sure.
Donnie Darko Perhaps not the proper tone for a Rifftrax. Already a comedy, but of a very different type.
Double Dragon hard to obtain?
Dungeons and Dragons Even though I would love to see this one done, probably hard to obtain
Flight of the Navigator hard to obtain?
Harley Davidson and the Marlboro Man hard to obtain
Mannequin hard to obtain?
Maters of the Universe hard to obtain? I know Netflix has it cause it's already in my queue
Maximum Overdrive Again a great idea but hard to obtain I'd wager
Monkey Shines Ditto
Purple Rain hard to obtain?
Prince of Darkness one of my personal favs, but I know it's kinda scarce already
Streetfighter: The Movie hard to obtain?
Superman 3 and 4 Hard to obtain? Are these even avaliable on DVD outside that box set?
Texas Chainsaw: TNG hard to obtain. and hey, I was the first one to even suggest it I think
They Live! I LOVE THIS FILM! But I think it's a bit hard to obtain
Trainspotting wrong tone?
Wall Street hard to obtain?
Waxworks hard to obtain (another I like and suggested)
Xanadu hard to obtain?
Zardoz hard to obtain?