Shin arc wasn't the only one with more fighters and depth.
the Nanto Goshasei (basically Raoh's journey towards the Last Nanto General) introduced many interesting characters. Though it was a shame they wasted Rihaku's character (the leader should always be the strongest!!!!) but the anime at least have him a actual fighting style.
Tou was also a great character that deepened Raoh's past as well, and she was given a bit more role in the anime. (it's insane Ken's Rage 2 doesn't include her yet they include Sayaka, Myuu, Leia, some minor commoner NPCs, etc. She should at least be present in Rihaku cutscenes. )
then you had the Gento/Celestial Emperor, arc in the anime they had FIVE GENTO GENERALS instead of 2/3. ( I heard that Shoki didn't even do much of anything compared to their anime version where they fought Kenshiro.) Also BOLTZ and TAIGA are waaaaaay better designs than Jakou's lame offspring.
Then in Shura arc there are notable fighters besides the 3 Rasho, like Nameless, Alf, Kaizer, Zebra, Jukei, Kuroyasha, Akashachi, Shachi.