A few players, and I thought it would be fun to have an "Arena" where you can vs. other payers.

And with the sections like:
"Max-state only"
"pets under a certian lvl"
"tournaments"
"1v1 (one pet)"
"2v2 (two pets"
"3v3 (three pets)"
"1v1 (people)"
"2v2 (people)"
"3v3 (people)"
"invite freind to battle"

And:
"same lvl people"

Here's the thread Paint

A "Last man standing" style rumble with a chest as a reward in the center could be fun. A free for all, maybe by level groups

or an "OP" like you mentioned

yes that too :) could be lots of fun

I favor this idea in principle. The important part would be how it was carried out.

Definitely going to be difficult to implement, and quite likely to be unbalanced (which only destroys competitive environments, not the idea as a whole)

Things like:
What gear sets people are wearing (LG, arena gear, etc)
whether or not pets are max-stat or super-stat, vs standard
turn based pvp can take "forever". just look at online games of diplomacy or chess and sometimes people take days to make each turn. Maybe a setting of time limit and people skip if they fail to do an action in that time. (I duel you! Time Limit: 90 seconds/turn!)
can you change your action after you have input and "end" it, and the other person hasn't taken an action yet?

Just remember the rules of internet pvp gaming:
1: There is always someone better than you. (Insert positive stereotype about Koreans)
2: There is always someone who spends more time on it than you.
3: There is always someone willing to spend more moolah on it than you.

All in all, I support the idea, though I hope Krinadon implements Multi-player PvE first (2-3 player dungeons).