Bulbasaur is Prof. Oaks ambassador for the Lab. You saw an episode of it on Pokémon chronicles, Ash never really used Bulbasaur after Oak took him. Only a couple of times.
I'm going to go through a list of Ash's Pokémon and see which he should get evolved, or the writers should of done a long time ago. And also name a few Sinnoh Pokémon he should get.
Current Team
Pikachu: Stay obviously, durrr.
Staravia: Evolve it to a Staraptor, then the writers can ignore bringing back Pidgeot sadly :(.
Turtwig: Stay the same, he seems like a Bulbasaur kinda style.
Chimchar: Evolve him, I hate him. He's been hurt by Paul, they should prove Paul wrong and get him very strong.
Builzel: He seems pretty strong already, but he's never had an evolved water Pokémon I think, correct me if i'm wrong. I say evolve.
Glisgor: Just evolved, loves to battle. Don't evolve (if it can, don't know much about Sinnoh Pokémon)
Pokémon at Oaks lab
Sceptile: Well it can't, not Digimon :P
Donphan: As above.
Swellow: As above. He does have a few evolved.
Corphish: I wish he did evolve in Hoenn, he made Ash look much like a rookie.
Glalie: I loved the fact he evolved, if only Ash used him more.
Torkoal: Don't think he can.
Bulbasaur: Nah, he refused to evolve and we've all grown to like him.
Bayleef: I say no, Ash should bring her back to fight in the Sinnoh league though. Or take her with Pikachu in the next region.
Cyndaquil: I remember someone saying he should of in a certain episode, when he saw a Quilava and needed evolved Pokémon to stand a chance. Should of then.
Tododile: I liked the energetic Tododile, his personality would of changed if he did. I say, let him stay as a Tododile.
Noctowl: The shiny can't, it's great Ash got a shiny and was a big member of the team. Was a great replacement for Pidgeotto/Pidgeot.
Heracross: He can't, but use him in the Sinnoh championships. He did in Johto, he beat a Magma. Come on!
Snorlax: He can't, but Ash will always use him when it matters.
Kingler: The only evolved water Pokémon, we all remember when he used Krabby and it evolved to win all 3 battles. We all loved Kingler, bring him back for an episode please!
Muk: Well, he can't. But should use the comedy of him taking out Oak from time to time, they don't anymore.
Tauros (30): They all can't :D. He used Tauros twice, should leave it like that.
Pokémon that can be called upon
Charizard: Well, was a Charmander. The best Pokémon Ash had, differant personalities with every evolution. Two with Charizard. We all get goosebumps when he returns. Hands down Ash's best, now especially since he's been flying on Charizard.
Squirtle: Squirtle squad? My point exactly. He'd only evolve if he gets into Team Wartortle.
Pokémon Ash should of kept
Pidgeot: Pidgeot didn't want to stay, what an idiot. He upset so many fans, damn those writers. He said he'd come back, it's very slim though. Maybe next time he goes to Kanto.
Primeape: He left to become a Champion, Ash was a huge idiot. A mental fighting type that liked him, fool.
Haunter: If only..
Pokémon where he done the right thing by letting them go
Raticate: Traded, but wanted Butterfree back. Too right.
Butterfree: To make a family with the pink Butterfree, a perfect farewell. They wont see eachother again sadly.
Lapras: It was an infant, needed it's mother, or Brock.. but he wasn't there.
Beedrill: Gave it to that annoying girl, but at the time. Beedrill seemed abit like a step-down for Ash's team.
Larvitar: I so hoped he could of just kept it, but needed it's mother.
I went on there, drifted off from evolution.