Please note, I did not make all of these ideas up. I am simply combining all of the suggestions for Spleef into one thread, and adding some of my own too.
1) Make it so that you have Speed 1 in Spleef, I guess it's just me but it seems right since you jump and walk quite slow, especially in spawn (If you know what I mean)

2) Anti-cheat seems to be a big problem. Make it so that it will only harm campers and not the people who actually play. It's easy to evade most of the anti-cheat, but sometimes when you have to jump onto a block that has no blocks adjacent to it and jump right after, you will tp back to the one with no block adjacent and fall because it disappeared due to the Anti-Cheat plugin. You can't really stop this one, I've tried forever looking for a solution, but there is none. Make it so that after three or so seconds of either sneaking or not moving, the block/s under you will disappear. What I mean by block/s is if you stand in the center of four blocks after breaking all of the blocks around you, then all four of those blocks would disappear after not moving or sneaking for however many seconds.
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3) Allow for power-ups, or something more exciting than running around in circles and breaking some blocks. Some would be maybe a Speed 1 if no. 1 isn't accepted, or jumping boost 2, or invis for 10 seconds, or even so that every block you break breaks blocks in a +. So like an-----X
----------------------------------------------------------------------------------------XXX
------------------------------------------------------------------------------------------X---------------shape.
Make these spawn every 30 seconds, meaning it will pretty much spawn once the majority of the players are out, so not everyone will have a huge advantage.

4) Allow commands (Already suggested). /msg /r should be alright, and /g too. I think any talking commands should be fine. If not, then maybe add a plugin so it says something like: "Sorry, this person is in a spleef match right now and cannot respond until the game is over!"

5) Make it so it's like one of those other systems where it makes you start with a fresh inventory, and not what you already have. When you go out of the game, everything appears back. Before you start thinking, "hell no", let me say something. Have streak bonuses that give more meebles such as the new terrarium plugin we have, and make them give special decor perks. Maybe you can get a glowstone /hat to wear when playing spleef (only), or have a wooden hoe called Skaros. It doesn't have to be streak bonuses either, it can also be like playing Spleef daily, or winning 50 matches, or by winning without breaking any blocks.

6) An achievement system would fit well with number 5, so that you can see what perks and achievements you have already gotten, then aim for another one. No explanation really needed.

7) Make the Spleef either layered, bigger, or wait for it... even both. It'll be more fun this way and more entertaining to watch.

8) This also fits with the number 7. If you want to watch in the stadium thing and it's layered, have a ladder to go up and down to different floors to spectate the people on that floor and how they're doing. Or better, make it so if you want to spectate you can fly with invisibility on. Basically, make the watching place seem more enjoyable.

9) Make it so it's kind of like Runner from that server, or TnTRun on Meepcraft. Except when you break it, the blocks will be completely destroyed, but when you run over it, it will turn one color, when you run over it again, it changes color again, and finally, it is completely destroyed.

10) Make Spleef it's own world. 'nuff said.

11) Last but not least (Until I come up with more), make Spleef so that you there can be multiple arenas. Now listen up, it's not going to be the exact same Spleef, but one could be teamed spleef, or one could be timed, or one could be speed spleef, etc etc. Some possible ideas are:
a) Non-Jumping Spleef
b) Timed Spleef
c) Teamed Spleef
d) Fire Spleef
e) Explosive Spleef
f) Snowball Spleef
g) Lose and Win Spleef (If you lose, you lose money. If you win, you earn everyone else's money)
h) Sudden Death Spleef (If you die, you lose your head and the items you have in your inventory)
i) Hunger Games Spleef (Collect special tools to better kill opponents.
j) Punch Spleef (You can punch players into pits while digging)