There exists many pokemon i wish Ash caught. Actually if i started to think of all great,alluring opportunities for not just Ash, but his traveling companions list would be very long. But to narrow down choices to select few in Ash case:

I always thought it would been cool if Haunter became his partner permanently never really having ghost type. Although based on Ash battle abilities and rush side back in day, its questionable if he would know howe to weilder correctly power from one of most powerful treated as almost unbeatable type. Back in early stages of anime, second was psychic being almost untouchable.

There was also Larvitar being suavely to see Ash develop entirely new side to his character. Showing mother like protective feelings and great deal of care. Earning Larvitar thrust, who jibbed away from his friends which made especially Misty irritated.

Some of more peculiar designed pokemon. Maybe Exploud or Cradily would been interesting in lest hand to say as well. Due to either delicate behavior, their movesets and abilities serving as excellent excercise for Ash to widen up his horizons and learn how to approach more demanding species as pokemon finding right battle style where he can work with them as team.

But one pokemon which especially picked my interest was fighting/steel type known as Lucario.

Mainly because of this pokemon having potential to serve as catalyst through which Ash character could grow in entirely new, creative way. Because of Ash connection to aura guardins with no pokemon being better at this than those from Riolu, Lucario lineage.

Because ever since i found out how Ash has aura abilities, it came of as rather endearing and gratifying discovery for me because of realizing how much writers could have done with him in that aspect. Once he started to discover in 8th movie but even later on through whole involvement of Riley Lucario how he has ability to control aura powers manipulating his spiritual energy and reading other pokemon minds and intentions. Bringing whole new dimension to Ash making him more interesting, mysterious to extent and unique. He was even linked to legendary aura guardian from ancient times in "Keystone Pops" episode.

If there ever happened to exist interest to exploit more on this side of Ash, in my opinion capture of Lucario would give extra mileage in adding novelty to his character, dive deeper in his thoughts and personality facing potentially dilemma,. Between responsibility to protect world as aura guardian or follow his heart achieving dreams. With Lucario helping him to find right answer and possibly learn from who Ash inherited this ability. Fleshing that aspect of Ash character through jackal pokemon and his excellent abilities in controlling aura using it in proficient way.

Pokemon was always sadly bag full of missed opportunities which had ability to be much better than it turned out to be.