Oh dear, if you need simple gamemode suggestions then here we go:
Come Nightfall
Essentially a freebuild, but after the sun sets zombie bots start spawning all over the map/server. Players are not given wrenches, or if they are, events are limited or will disable themselves for X amount of time after being activated. Weapons or "supply boxes" will be airdropped at random within 100 meters of a player, and they start with a weak melee weapon only.
Your bricks are cleared upon death, so each life is starting anew unless you lived in someone else's base and they didn't die.
Zombies damaging bricks is optional.
Freefall
Start in a high spot in the sky. You will start falling after the game starts. Once you press your jet key your jets will activate and will stay activated until your velocity is at zero.
The goal is to fall as far as possible and survive by activating your jets the latest. Score would be determined by health and height off the ground when your velocity reached zero.
Essentially it's that GTA freefall challenge thing in Blockland, but with random heights that everyone starts from as to reduce just timing yourself for the best score every time.
Bullet-time Assault
Run through a map with bots, and you and several other players (if the server has multiple players) must reach the end.
The catch here is that the server will have its timescale slowed down in specific areas, mostly for combat. If you cannot kill all the bots in the "bullet time", then you must face off against the remaining enemies in a normal timescale. Hope you're good at dodging.
Courier
Two bot factions are battling against each other. You are delivering an item to a target on the other side of the war zone. Try to survive and deliver your package - but be warned that other players may try to take it from you and deliver it themselves!
Either one person has a package each, or you can pick up infinite packages from the starting zone, but only can get one at a time yourself. You will get different packages depending on your score (eg. deliver a hammer at zero points, deliver a wand at 1 point, etc). This allows you to only pick up packages you do not already have yourself, and prevent killing people at spawn to steal packages.
There ya' go, 4 reasonable gamemodes. I assume they're reasonable at least, especially the first two. Change 'em up as you see fit.