Here is the problem: you treat levelers and PvPers as two fundamentally different groups, and you discriminate. You believe levelers are playing the game the "correct" way and PvPers are "leechers" who are not afforded the same basic privileges as levelers. I suggest you correct your mindset immediately.
There is no "correct" way to play this game. Notice that FallenSword is, indeed, a game, which means that if you're having fun, you're doing something right. Whether you enjoy leveling up, PvPing, GvG, farming plants, farming fragments, merchanting, buffing, Arena, Titan hunting, SE hunting... if you're having fun, there is no reason why you should be accused of playing the game the "wrong" way. If PvP was a wrong way to play, HCS would have never added PvP to the game. Think for a second... the reason why content exists is because it is meant to be played! The reason why PvP exists is because people are meant to PvP and enjoy it!
Now that we've got that out of the way, the crux of your argument is that PvPers should suffer because they make levelers suffer. Retaliation. However, stop thinking about groups for a second, and just consider the individual.
The leveler spends stamina and he gains XP and gold.
The PvPer spends stamina and he gains gold and PvP Prestige. However, there is a chance that the PvPer will be bountied, resulting in a loss of levels and gold.
You see, there is already a fundamental disadvantage to PvP. Like I said earlier, there is no "correct" way to play the game, so if we accept leveling and PvP as equally valid playstyles, you'll see that the PvPer is already being punished for basically playing the game.
Do you want the system to enforce fairness? Then we conclude that the fairest system is to allow monsters to punish levelers. After all, both PvPers and levelers are trying to play the game, the difference is that PvPers are being punished for their playstyle. Shouldn't levelers have to suffer in some way - periodic loss of gold, XP, maybe levelers should randomly be incapacitated for a few hours, unable to spend stamina. Woah there, I bet that idea doesn't sound very good to you.
Now, I understand that you will argue that levelers are hurting nobody, while PvPers are hurting another group of players, so PvPers should indeed be punished for their playstyles. Perhaps that is correct, but your suggestion is that levelers can inflict equal retaliation to PvPers. That idea is utterly unfair to PvPers when you consider the costs/benefits.
Here is the current situation: PvPers spend stamina, lose XP, gain gold. Each player decides for himself/herself whether the trade-off of more gold for less XP is worth it... those who accept the trade-off become PvPers, those who do not become levelers or Titan hunters or whatever. The important part is that there is a trade-off - some players will naturally value gold more, and so there are PvPers. Of course PvPers are going to value levels less than a leveler... if a PvPer valued levels just as much as a leveler, the PvPer would have never chosen to PvP! You claim that it is somehow unfair that a PvPer will not care about the levels lost, but that's the way the system has to be.
On the other hand, let's say that a PvPer steals 1 million gold. Under your system, the leveler is able to retaliate equally, so the PvPer also suffers 1 million gold worth of damage in some way... maybe the PvPer loses enough XP so that the XP lost is valued at 1 million gold to the PvPer, or perhaps the PvPer loses stamina which the PvPer values at 1 million gold... In any case, you have designed a system so precise that the leveler can retaliate equally. Now, what does the PvPer stand to gain from PvPing? Absolutely nothing, there is no more trade-off... the PvPer spends stamina, gains gold, and loses exactly what he gained. With your suggestion, there will be no such thing as PvP... absolutely no one would PvP because there is literally nothing to gain.
Then here is my personal suggestion: if the PvPer must lose everything he gains, then the cost of retribution should be everything the leveler gains. In other words, if a PvPer steals 1 million gold, the leveler is allowed to automatically deduct 1 million gold from the PvPer's bank and holdings. In exchange, the leveler will no longer gain XP or gold from creatures. How does that sound to you?