Well, provisionally let's go with this, anyone feel free to pitch in with their ideas. This is all IMO.
Apologies for repetition of points, just to get it all in one post:
what adjustments should be made (be specific) and why?
Change the damage to 20.
Change the energy usage to 25.
Change the ammo to 15.
The snipers strength is the projectile speed (instant) and the ability to damage someone from a spot of safety. A good sniper can damage anyone within their sight immediately. This is not fitting in an otherwise completely prediction based FPS+Z.
In it's current state, the sniper drains all energy after a shot. This was intended to put the user in a state of weakness, however simply makes users restrict themselves to safe situations eg. standing around their base or so far away from the target that retaliation is easily avoided.
If the damage and energy usage were scaled down together, players wouldn't be restricted to these situations. It would be viable to use in every position and more often in flight if the player is smart.
The skill requirement of the sniper would be increased, closer to that of players using other weapons like the disc launcher, helping overall balance. The use of No-scopes would be promoted. A 4-hit sniper chain would be admired instead of disdained. The sniper would viscerally *feel* better to use, and be less of a guilty pleasure.
A player would often not be able to simply rely on the sniper to kill the enemy. A variety of weapons and tactics would have to be used.
Energy <-> ammo and why?
How much energy usage <-> how much ammo and why?
See above. Energy and ammo should be consumed, to promote the use of careful, thought out shots. There would be an opportunistic gap in the flag defending of a sniper as he goes to the inv. Also because well... It's an Energy Rifle.
If ammo, reload times and why?
1.5 seconds seems reasonable. With a perfect body shot chain and no other energy consumption, 6 seconds to kill a light, 7.5 for a medium, 15 for a heavy.
Headshot, what to do with that and why?
Perhaps HS=30 damage. 3 hits for a light, 4 for a med, 7 for a heavy. Doesn't matter too much as headshots are more often than not luck. Can't remember who said that thing about headshots being a reward for nearly missing (Might have been Bins.)
How to prevent the sniper from becomes useless (remember, I'm in for tweaking weapons but you can forget it right of the bat that I am going to allow weapons to become downright useless)?
If anything, while the sniper would become less powerful, it would be more 'useful.' The sniper would have an increased variety of situations in which it can be used effectively. Though at first it might seem quite weak, a higher skillcap and less stigma means more people will train and become better at it.
How to not make a heavy an OP counter vs snipers?
Gonna need help with this one.
I am tempted to say 'use a different weapon.' The sniper will still have the advantage if the heavy is far away enough, if not the disc launcher may be better to fall back on.
What about amplifying the damage when hitting someone in the bac0 and why?
What about sniper damage to vehicles and why?
What about sniper damage to buildings and deployed assets and why?
Could be interesting functions, but personally I see no outstanding reason why these values should be any different from the norm.
I am a fan of disabling the current sniper until 5v5. However I think it would not be necessary with this idea.
All I would ask is what happens if you are using a sniper and the server population drops below 10?