There have been no major changes to battle mechanics, or overworld mechanics yet. All pokemon are intended to be available, except legendaries. Not even postgame. There is a large variety of pokemon available, even from the start, and all are intended to be viable. Though, this will take time.
Mightyena: Lvl 17 odor seluth | Lvl 18 Body slam | Lvl 22 Crunch | Lvl 25 Signal Beam
Onix: Iron tail Lvl 45 -> 25 | Rock throw Lvl 13 -> 10 | Bind lvl 9 | Earthquake Lvl 35
Swampert: Earthquake Lvl 37 | Sludge bomb lvl 48
Marshtomp: Lvl 19 water pulse
Grovyle: Lvl 19 leaf blade
Combusken: Lvl 19 flamethrower
Mightyena: Lvl 17 odor seluth | Lvl 18 Body slam | Lvl 22 Crunch | Lvl 25 Signal Beam
Onix: Iron tail Lvl 45 -> 25 | Rock throw Lvl 13 -> 10 | Bind lvl 9 | Earthquake Lvl 35
Swampert: Earthquake Lvl 37 | Sludge bomb lvl 48
Marshtomp: Lvl 19 water pulse
Grovyle: Lvl 19 leaf blade
Combusken: Lvl 19 flamethrower