A few suggestions I've thought of over my time of playing Roat Pkz is..
Replacements:
Instead of Chaotic Maul, replace with an Abyssal Bludgeon (5K PK Points)
Instead of Chaotic Rapier, replace with an Abyssal Dagger (3.5K PK Points)
Instead of Chaotic Crossbow, replace with an Armadyl Crossbow (5K PK Points)
Add-Ins:
Clue Scrolls, hey, this could be a fun thing that people can buy for PK Points or have a rare chance of getting from a kill from a boss or player..
Blowpipe, OP but fun and another PKT sink none-the-less..
Falador Drop Party Room: This could also be another fun thing to do..
DiceBags/Black Jack Tables, the losers item would be taken off the account, and put onto the winners, and both players must have an appropiate amount of money/PKT to do so, that or an NPC which has it's own gambling system.
An exclamation mark thing for when someone tries to change rules/change their trades to prevent being scammed..
Wards... Odium Ward and Malediction Ward..
Bosses:
Zulrah: have this monster be a donors only monster, drop Blowpipe, Magic Fang/TSOTD, Serp Helms
Jad: Another way of accumulating the Fire Cape
Bandos: No time like the present to make Bandos something you have to acquire.
Armadyl: Same as Bandos, but with additional drops such as Armadyl Godsword/Armadyl Crossbow
Corporeal Beast: Why in the fuck does Chaos Elemental drop Divine when Corp isn't hard to code?
Suggestions from others:
RawrDaTank: Having bosses drop untradable pets