Ash pretty much did everything he will ever do in terms of character development at the end of Johto. Defeated Gary, nearly won a conference, showed his extreme ability to help any pokemon through compassion with Larvitar. He effectively did little nothing new of consequence between that now.
DP because Ash did little to nothing better in BW/XY/SM
-Defeated a trainer's Darkrai at the sinnoh league,a feat that no other trainer did before him (Tobias)
-Defeated the most skilled trainer in the entire pokemon anime at the sinnoh league (Paul)
-Made it to the Top 4 of the sinnoh league
-Grew into a battler that surpassed that of Gary,Harrison,Tyson
-Developed a balanced strong team in sinnoh
-Used his reserves at the sinnoh league
Ash accomplished something effectively similar to all of that in the Johto league.
Ash came extremely close to beating Harrison. Harrison probably would have won the Silver Conference if his star pokemon Blaziken hadn't been too exhausted from the match with Ash. So Ash was arguably only beaten by the strongest competitor in the league, just like he was in Kalos, Sinnoh and Hoenn.
Ash defeated Gary, capping off a victory against a rival who he had both a close personal relationship and contrasting personality and training style towards. That was followed by a show of mutual respect and the two parted ways on friendlier terms. Just like Paul.
Ash used a mixture of Kanto and Johto pokemon to great effect in the Silver Conference. Including the semi-released Charizard and Squirtle. His team was diverse and he had built a strong roster.
Ash defeated Darkrai, but that's really just a case of taking down something with bigger numbers. Its not really the same as a sign of personal growth.
Ash beating Paul was a perfect karmic reward for their contrasting treatment of Chimchar. But the way Ash trained Chimchar certainly was nothing new. Ash always selflessly looked out for and believed in seemingly-pathetic or useless pokemon and it usually payed off for him. Lapras, Charmander and Cyndaquil were all bullied or neglected for being weak before Ash showed them unconditional love and support. It paid off in all three cases. That's basically always been Ash's whole deal.