My suggestion is basically getting upgrade points as you upgrade to an bigger level. Players will get 1 upgrade point per level,.
- Armor- You can upgrade 5 percent of armor when you respawn by 1 upgrade point.
- Fighting Damage- 5 percent better damage for someone that upgrades his fighting skills (( This will be avalible from lv5+))
There can be many things to upgrade with the points, if you got any suggestions that you can add feel free to comment it.

Then increase HP for players..

No support, it would be completely unfair for lower level players.

No, just no. We're not a supernatural themed roleplay server.

There is a difference, low and big. They are not the same, there is why the oldest players should get more stuff. It's not about being fair. The new players will play more because of this and they will reach level five faster. The active players need to be rewarded and this is a good way, if you're active you get stuff.

Everyone should be equal. This is like saying that black people should not be treated correctly because they're black. It's WRONG.

I am not saying that. What I am saying is.
Taking a 15 year old boy, and a 21.
The 15 year old  guy doesn't have the power and the knowledge as a 21 men.

Then you roleplay that. Simple as.

There is tough and weak people out there. If you want to development your character as weak don't use the upgrades.