While I'm not big into PVP...

I wouldn't mind this as a Wolves Den mode.

Guildies and I on RO used to go into an empty PVP map (they took copies of maps of the original main cities and set them as PVP zones) and duel for fun or play a deadly hide-and-go-seek ------
or just completely mess around for fun. Sometimes that messing about wasn't even dueling - like a STR/VIT whitesmith with natural fire def wearing all fire resistant gear guildleader and my Agi/Int Hindsight Sorcerer soul linked with firebolt set for hindsight.... and just wailing away with my book on my guild leader for shits, giggles and seeing crap tons of level 10 firebolts beating the snot outta him (and then a guildie decides to be funny and start something while we're distracted - and we promptly put them in their place... but it's all done with laughs at the end from all of us). Or just, "Okay I'm bored now, someone kill me *Everyone slaps the person out of the PVP zone all at once*" (as you're ejected from the PVP if you die twice)
Not a fan of parties coming in and steam rolling us on sight, though =p
But a duel offer requirement would alleviate that.

Every now and then I wish I could do the same thing. My MNK vs my old RO guild leader's NIN or my WHM vs whomever. or just poke around for shits and giggles. Not for bragging or 'better than thou' rights, but more like a friendly arm wrestles or sparring.

Worst I can see from implementing a duel system (with acceptance system) is harassment of the five year old child kind;

"omg u suk fight me"
"No"
"omg thats cause you suck so u wont fight me lolololol omg lol u suck"

Bad typing and all.