Dicing/Flower: I'm the only person so far to endorse gambling being added, (Now Opi too.) I love the way it's coded already. It would be fair, simple, and fun. You could even take it a step further and make it Player VS NPC. So nobody is getting someone else's money and it acts as another money sink. It would be cool to add server announcements for players that win above a certain amount too.
Cost of Death: Out of the options provided I'd pick just paying coins. Another option would be dropping your inventory along with a fee from Death to respawn.
PVP: I've never really been a PKer. I know that's what A LOT of the osrs / ps players enjoy, and they have 100 servers for that. It would definitely be a challenge to add enough incentives to entice people to risk it. Unless you do apply the same cost of death as above and make it fairly safe. I've been on other servers that have safe pk'ing with a chance to drop statues that sell to a NPC for gp, but that almost always results in people farming it.