First of all, your current motherboard supports a 9700K.
I see no need to change out the motherboard.
There always is a limiting factor in games.
Usually the cpu or the graphics card.
If you favor fast action games, you may not see much improvement since such games will be graphics limited.
OTOH, if you favor sims, mmo or strategy games, you will do well since they are cpu centric.
Multiplayer games will benefit from more cores.