Some of them were pretty memorable, like Duplica (though she came back in Johto), AJ, Giselle, and the hippie with the Pokéflute to name a few. They were probably better developed one-shot characters, but there were those whom you could tell were just made for that episode, and thus flat as the cel they were painted on (the brothers in the Eevee episode, the samurai and the biker punks, for instance). As a kid, I believed in Ash when he said "one day we'll see each other again".
I was such a fool.
Do I wish they would return in a future episode? Yeah, a few times whenever they came to mind I wished they would show up again, but it only happened at least three times when the plot called for them (Suzie, Todd and Duplica are the ones I can think of at the moment). Even then, my expectations for them weren't really as great, and it slowly made me realize that one-shot characters should just remain as one-shot characters unless the writers could give them a proper re-introduction and keep their characters the same, but add a little more development to them. That's why I feel Duplica is probably one of the few one-shot characters I really liked in the whole series. She was rather unique in that she was a copycat (probably based off Copycat from the games now that I think of it), and in her debut episode, they used her copycat talent to thwart Team Rocket (and poke fun of them), which you don't really get to see much in an episode, a Chekhov's skill--especially one that was a character's signature trait.
The Samurai was actually a well-developed character with a deeply hidden backstory and some really dark roots.
I probably don't remember him very well, then. Yeah, he's hard to forget, he was probably the first one-shot character in the series, but he just wasn't a personal favorite of mine. I wonder if he ever stopped training in Viridian Forest, though...
I thought I was the only one who didn't like Samurai and I also find Giselle to be overrated as well. She is pretty, yes but that does not make one an interesting character. But yeah, I agree with a lot of your choices from your earlier post. AJ was a huge favorite of mine and I would have loved to see him in the Indigo League.
COTDs is one of the few advantages the anime has over mangas or fan fics. Because, well, they're more realistic. You can't honestly expect going through the region with only meeting your rivals, your mom, your long lost childhood friend blah blah blah. That's why I liked DP. It had a deeper depth of realism in the form of COTDs and fillers, as well as a darker theme sometimes.
Doctor Who has longer episodes, allowing more developed plots/COTDs. The older Doctor Who series was actually split into mini-arcs. I wonder if that wouldn't work for Pokemon.I think so too... in concept. Unfortunately, a lot of CoTD episodes have a predictable vibe to them outside of certain standouts and they could've been used to develop the main characters more. It just comes off as "meandering" at times and it seems like the writers are afraid of major plot progression. The concept is good as Doctor Who shows but in Pokemon, they started to get a bit... bland at some point and for me, it wasn't one of DP's finer points save for a few better episodes.