Here are some cool themes to lay the ground work for you maps. 
Ancient Ruins
Have your players explore ancient ruins. They could collect research from the ruins or maybe they got there completely on accident and are now being attack by plants. Its up to you. 
Space Investigation
Ever since GIMKIT creative was made there have been maps based on the GIMKIT mode ONE WAY OUT. Now to play on that idea you can create a story where a group of GIMS investigate a spaceship to uncover its secrets. You could even add an onboard AI that can give hints using a button, command table, and a popup.
Moon Base
I dont see people talking about this but for those who dont know there is a type of terrain called the space rock. The space rock terrain is the moons surface. So why not make a moon base using this type of terrain.
Survival Island
A popular theme among GIMKITers is the Survival Island. There are many things you can do with this theme such as, a battle royal, story, hide and seek. You can even pair this theme with the ancient ruins theme.
Medieval Fair
With the new DLD update came a spike of new medieval style maps. To contribute to that here is a Medieval fair to put into some of your maps. Some additions you can add to your medieval fair are some food you can buy from the stalls or even a indoor circus section with minigames you can play inside. You can eve make a market place to sell items or to buy items.
[Circus idea by: @Blackfox45666]
[second picture and market place idea by @gimmaster12345]
Snowy tundra
Based on SNOW BRAWL and SNOWY SURVIVAL we have the Snowy Tundra theme. The Snowy Tundra is hopefully self explanatory but what it is just a snowy area. You can even pair this one with the Ancient Ruins theme, and if your feeling cool you can even pair this with the Medieval Fair.
Under The Sea
This theme Is underwater. The main terrain yyou will use for this is the sand terrrian. You can create the underwater affect by using blue colored barrier devices with no collision. An optional touch is water physics. when you enter the underwater area your speed is cut in half. however you can aquire an items that will put your speeed back to normal. For the fish you just make a item spawner with fish and cover the bottom portion with a plant.
(theme concept and picture by max1)
Infernal adventure
This theme is fire based. meaning your going to have to add that fire emoji 
. To make this as realistic as possible use the dark colored terrain to add that feeling that your surrounded by ash. For the lava use red plastic and for an effect use red barriers that have no collision on the top layer. A game mechanic you can add to this theme is super heated rooms. These rooms will require your players to get an item or ability in order to progress through these rooms. To make the super heated rooms you will need invisible lasers that turn on and off every second. (you can stop using these after the first area to conserve memory.
(Idea and picture by @Aubec7 )
I may update these in the future. If you have any theme suggestions, type a reply below stating the theme, and a picture of you using the theme I will give credit and add them to the Post. You may also give me things to add to already existing themes, and I will also give credit for that.
Be sure to like to show if you found any of this useful, Have a wonderful day. 
!n