In FFA there is no goal or something, so it is kind of boring. Therefore I have an idea about FFA.
/target:
When you use this command you will be in target mode, and you will get absorption 5 and glowing for 10 minutes, so you are hard to kill and players can find you. This command will make you a target to everyone in the game. If someone kill you, he will get 5 points as the reward.
When you are in target mode, you will get 2 points each kill, your name tag will become red (so is your glowing effect). target mode will last for 10 minutes. After using it you need wait 20 minutes to use it again. If you die or log out you will exit target mode. If there are 3 players in target mode you can't use this command, you need to wait until one of them exit target mode.
Also In kit selecting room there should be a text in the air says:
Do /target to be a target for 10 minutes, you will get 2 points per kill.
Kill a target to get 5 points!
When someone become a target, there will be an announcement in chat:
<player> has become a target! kill him/her to get 5 points!
When a target died, there will be an announcement in chat:
Target <player> is died! do /target to become a new one!
If the target is killed by someone:
Target <player> is killed by <player>! do /target to become a new one!
Please post below to let me know what you think about it!:D