HI, I'm new and I've played the game and noticed that the game is a it similar to gunz(since I'm guessing it will be fast paced) If you dont want to read everything then just look at the last 2 sentences
1. Before getting into the anything, you should introduce a knock down system where if enemy gets hit by an attack he gets mini stunned. Also, when low hp they should get slowed so that they don't run away the whole time.
2. But what I think what you should prioritize first is taijutsu, a basic fighting system in a game.We already have some jutsu, now you should pause developing the jutsus and make a hand to hand combat system so that everyone doesn't have to run around like drifting lamborghinis and and just luckily hitting someone. So if someone wish to not plAfter ay smart and just go crazy they can. Just a normal fighting system to throw combos around. You can add substitution if you want or maybe later.
3. And then go back to ninjutsu, so  you should design the jutsus in a way so that they can clash, like 2 fire breath instead of hitting each other they clash(and go to the button pressing mini game- if no one can win then the first person that runs out of chakra gets hit)
After that making decent amount of jutsus maybe add a type effectiveness system like pokemon .
And then you could go into unique details like puppet, summoning and eye power etc
Sooo, everything summed up 1.Fix>2. Taijutsu>3.Some Ninjutsu>Some Genjutsu
This will be the basic foundation that you can develop on.