Variations in Implementation:
aaaaa Adding 2 grounds, namely Rectangle w/ a hole & Circle w/ a hole
bbbbb Adding 2 grounds, namely Rectangular hole & Circle hole
ccccc Adding an adjustable, a fixed, or a non-adjustable (which adjusts depending on the ground's dimensions) hole option for grounds

Advantage(s):
+ Reduced ground consumption. (e.g. being able to create 1 rectangular ground with a hole rather than creating rectangular grounds, being able to create 1 circle with a hole rather than creating 2 circles and 16 rectangular grounds)
+ Less effort to adjust an amount of grounds.
+ Probably encourage users to create more portal maps. (It seems to me that portal maps are uncommon)

Option A
~ 2 new grounds will be added: a Rectangle with a rectangular hole & a Circle with a circular hole.
~ These holes are non-adjustable and will adjust depending on the ground's dimensions or;
~ If possible, these grounds will have an exclusive property which can be manually adjusted.

Option B
~ 2 new grounds will be added: a Rectangular hole & a Circular hole.
~ These grounds will always be on top of other grounds and will prevent collision with mice, objects, etc.
~ These grounds may imitate the color of the ground under the one it's on top of (imitate the color of the second ground under it);
~ Have the default background color of the entire game, 6a7495 or;
~ Have an adjustable color option.

Option C
~ Rather than adding new grounds, a new property will be added for grounds: Hole option.
~ A hole will be applied to a ground if the Hole option is toggled on. If toggled on, a rectangular ground will acquire a rectangular hole while a circle will acquire a circular hole.
~ If possible, this property can also be adjustable which changes the size of the hole.
~ This option can be exclusive to the Rectangle and Circle grounds or accessible to all grounds.

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I would actually prefer to prioritize in implementing one of these options to Circles since it might have an impact to map making. Rather than creating 18 grounds just to make a Circle seem like it has a hole in it, you could reduce the number of grounds necessary to a minimum of 1-2 grounds which makes about a difference of 16 allowing the map maker to further showcase one's creativity and ingenuity.

Refer to the following images:



Glossary:
Circle - a circular ground with adjustable color
Rectangle - a rectangular ground with adjustable color
Rectangular grounds - grounds of rectangular shape; all grounds except Circle