I tried making a list some time ago what Pokemon each character would have if they managed to catch everything they tried to catch, kept evolving them, and didn't release them. I only got to the end of Hoenn, but Pokemon Ash tried to catch but didn't get are Pidgey, Weedle, Rhyhorn, Fearow, Scyther, another Weedle, Claydol, and he also tried catching a Corphish a couple episodes before he got his, don't rememeber if that was the same one. So if he's to catch something, maybe one of these? But I'd rather see him use the ones he has, maybe finally evolve Gible.