Door research
Let’s begin with the most easily way to represent a door: A door that lets you navigate down in the depth layers but without the ability to come back.
These doors are beautifully represented in mario worlds:
I love this one partiular because it embeds the input prompt in its design:
Now let’s move on to another type of door: the door between two adjascent rooms in a side scrolling view.
Usually the best way to make sure your door looks like one is to use perspective:
Another perspective:
However some games choose not to have perspective for different reasons.
Entirely ditching perspective is dangerous because the door is more difficult to represent.
The opening animation is not really a problem and games have used a number of solutions in the past.
For example the classical ornate door in Castlevania:
Or the uplifting door in MegaManX:
The real problem being to identify the object as a door before the player runs into it!