Me personally I would like to see the following:
Golgo 13 x Punisher
Blade x Hellsing
Tenchi Muyo x Green Lantern
Guyver x Ironman
Wolverine x Afrosamurai
Cowboy Bebop x Batman
Naruto x Bleach but done like those Marvel x DC Amalgam Comics where rather than both characters existing in one world, they combine two characters into one brand new character ie Batman+Wolverine=Dark Claw, Superboy+Spiderman=Spider Boy, etc.
Squid Girl x Hayate no Gotoku
Samurai Pizza Cats x Zettai Karen Children
Mitsudomoe x South Park
Yakitate Japan x Yumeiro Patisserie ~ "the great bake-off"
Eureka Seven x Nadia: Secret of the Blue Water ~ "my girlfriend's got problems"
Dirty Pair x Zettai Karen Children ~ "some collateral damage is expected"
Zatch Bell x Sekirei x Kurokami ~ "I choose you!"
Magical Girl Lyrical Nanoha x Mahou Shoujo Madoka Magica ~ "Sailor who?"
And just for my entertainment: Galaxy Angel x Keroro Gunsou (Sgt. Frog) ~ I just think it would be the most insane, chaotic, nonsensical (but not absurdest) crossover ever.
The Best List is here to run down the best films to capture the essence of the Vietnam war.