Staying at 15 would be exactly what Naruto is doing right now in the Shippuuden episodes. He's still Genin level but has the power and mind of a Chuunin, even though everyone in his age group(yes including Sakura, she is a chuunin) is a Chuunin and he isn't. But Naruto also has the deal with the fact that he is still a lowly Genin and the rest of the people are way ahead of him.
The same applies to staying at level 15. There are disadvantages and advantages that even themselves out. As there is only two people that have decided to stay at level 15 that have access to Hoshi and the good mounts. The rest are just acads who got bored one day and decided not to slay Oro. Just let them live and not limit attack and defense. 
i agree <.< but there should be the event lebis suggested about having oro attack the people who stay at lvl 15 for too long...
and i just dont want there to be a pvping against HIGHER ranks limit.... lower is okay....
Exactly. Is Naruto not sought out on occassion by those stronger than himself? Sasuke was as well.
I think the stat bonuses should decrease from 2 to 1. Or, the frequency of those certain events scaled down to
1 reward per day (if you encounter them more often, you just lose with no opportunity to gain).
If I recall correctly, the first three gamedays that someone starts out, they have immunity from PvPs "from the Gods".
How about a similar PvP range immunity once the "Gods" protection is lifted? What I mean is that you have your normal range of who you can attack. (5 DKs below you and "Your DK + 10% DK" above you) That upper end of the range is not a ceiling, it just marks the range of shinobi that you can attack at will.
If you go above that, you get a message indicating that attacking that high removes your protection and you then become fair game for EVERY player in shinobi, regardless of the other persons DK level. (I have no idea how hard this would be to code. It seems like it would be rather tedious.) It just seems bit unfair that these players that stay there forever can take shots at a higher level person, but that person cannot reciprocate. You want to flex by seeking out stronger opponents? Then you forfeit the protection of being strong but at a lower rank.
I also like the Oro seeks you out once you pass a certain EXP threshold - or it could be based on "it is day number x for you" and when that day hits a certain number (100?), Oro seeks you out. Obviously, if it were to be an EXP threshold, it would be different depending on your DK level. But if Oro gets stronger based both on your DK level AND your other stats, then it should be no problem for those people to lose on purpose in order to stay at level 15 and keep all their stats (minus a % of EXP).
