Ash
Bulbasaur: It won't come back. It has a duty of keeping peace at Prof. Oak's lab. If it left then chaos would break out back in Pallet Town.
Kingler: Why would it come back? It really had no relationship with Ash. Out of all of his pokemon Kingler has the smallest chance of return.
Muk: It has grown more attatched to Prof. Oak than it has to Ash. The whole "glomp gag" is to priceless to lose.
Snorlax: Snorlax is a LAX pokemon. Sure it's good at battling, but I'm sure it likes living in peace at Pallet more than a life of battle. I'm sure Ash knows this too.
Heracross: This is one pokemon I can see coming back. Plus you can actually catch this is Sinnoh.
Cyndaquil: There's no point in bringing back other starters if you've already got two in your party.
Bayleef: Same as above.
Totodile: Same as above.
Noctowl: Ash already has a regional bird.
Donphan: He's had this pokemon since Johto. The only pokemon he needs to hold unto this long is Pikachu.
Corphish: He's already got a Buizel. Why would he need two water pokemon?
Sceptile: There's no room for development. It's fully-evolved, has no unsolved character issues. This pokemon has nothing more to gain. Bringing it back would actually make it worse.
Torkoal: Ash already has a perfectly capable fire pokemon. Plus it would hold everyone else back. It can't take a hit without crying.
Swellow: Ash already has a regional bird.
Glalie: Maybe if it was still a Snorunt, but no now. Already evolved.
Primeape: That fighting guy is getting his money's worth off of it. Ash ain't gettin it back.
Squirtle: It's a firefighter. If it leaves, the other squirtles will eventually dispatch, and the civillians will all die in fire.
Charizard: Let's get this straight. IT WILL NEVER COME BACK... PERMANENTLY.
Butterfree: It's getting it's thang on with it's wife, and raising a family.
Pidgeot: Ash is never coming back for it. We can dream. We can dream.
Lapras: It was reunited with it's family. Why would Ash tear that up.
Brock
Geodude: This could work.
Steelix: This could work.
Marshtomp: Nothing more to gain. Evolution doesn't really fit well with Brock's Marshstomp.
Ludicolo: This pokemon stunk from the getgo.
Forretress: This could work.
Crobat: MIGHT work, but with Team Galactic with their bat pokemon...
Team Rocket
Cacnea: Maybe later, but seeing as how Ash never came back for Charizard (permanently), I doubt James will.
Chimecho: The writers probably forgot all about Chimecho.
Lickitung: This will NEVER happen. Benny is long gone. Plus Jessie would have to give back Wobbufet. Now think about that for a second.
Arbok: It's protecting it's wild brethren. It leaves, they get back in trouble.
Wheezing: Same as above.
Dustox: Same as Ash's Butterfree.