Sneakers in FOnline used to be scouts mostly, while ambushers/campers still can be used in combat, simply it doesn't work properly now.
First SD should be possible with almost any weapon, but have different effects on it + max range of SD possible. SneakSkill%/15 hexes range must be max, where SD can be triggered, in other words 20 hexes with 300% sneak. And of course remove that silly auto-crit.
As for weapons:
2H ranged bursting - x1.0 dmg mult (no bonus). If you're in sneak mode or target can't see you: +10 to crit roll in case of critical.
2H ranged single (except for bazooka and flamer) - x2.0 dmg mult. If you're in sneak mode or target can't see you: +10 to crit roll in case of critical.
1H ranged bursting - x1.5 dmg mult. If you're in sneak mode or target can't see you: +20 to crit roll in case of critical.
1H ranged single (except for grenades) - x3.0 dmg mult. If you're in sneak mode or target can't see you: +20 to crit roll in case of critical.
Any unarmed/melee weapon - x6.0 dmg mult. If you're in sneak mode: +30 to crit roll in case of critical and instakill roll*
*If you're holding one of: power fist, mega power fist, cattle prod, super cattle prod, ripper or supersledge, then you're doing your ST roll against target's EN roll for instakill. Example: you have 10 ST, target has 6 EN, you're rolling random number between 1 and 10, and enemy between 1 and 6, if your roll is higher than target's, then target is instakilled. Would be nice, if instakill will work like death from poison/SS effect, setting HP to 0 and actually killing target without gore animation and armor breaking, not just KOing.
Silent death will trigger:
1) From any direction, if target can't see you, even if you're not in sneak.
2) From behind and back-sides, always, even if you're seen and you don't need to be in sneak.
3) From any direction, if target is blinded and you don't need to be in sneak.
4) From any direction, if target has less than 0 APs or lying on ground and you don't need to be in sneak.
Anti silent death/sneaking:
Your carry weight % capacity proportionally decreases your sneak skill, example: 0/45 weight - no penalty; 15/45 weight - your sneak is decreased on 1/3; 45/45 weight or more - your sneak equals to 0% or even negative if you're overweighted.
Bluesuit equals to CA/BA in terms of penalty to sneak skill.
Less penalty to sneak or negate penalty to sneak for 2H SG weapons.
Cautious Nature perk gives immunity to Silent Death perk, or PE roll (10 PE - immunity) that SD won't trigger on you. Also that +PE bonus for encounters can affect anti-sneaking FoV. Requirements can contain some sneak skill as requirements in additional to Outdoorsman, like 50%.
Bonuses from walls and obstacles should affect you even if you have base 300% sneak, in other words bonus should go over 300%, to make sneaking near walls/behind obstacles always better. With carry weight penalty sneak won't be so static (always max) like now, Ghost perk could be really useful regardless of your sneak skill.
Build: this way SD sneaker one-on-one isn't good, he'll only good against running away targets or one-time assault from sneak, while in team, he can be mobile in combat and run from side/back side direction to deal additional damage to enemies, while these enemies fighting with his friends, also SDers can assist-kill KOed and blinded targets. Also he doesn't need to run away and resneak after each attack to use his build effectively, as he will be able to do sneak attacks even being seen, just under right conditions, also sneak attacks won't be so overpowered as now in term of effect + more diversity in SD builds: you can even make unaimed fast shot SDer, burster SDer, sniper SDer, adequate unarmed/melee SDer.