Since the Grounded twitter page recently presented a question about who the "next new neighbor" should be, I think now would be a good time to suggest a fifth playable character. Perhaps a customizable character?
The customization categories could be split into Gender, Head, Hair, Clothes, and Voice.

Gender: Lets the player choose between a boy or girl. Determines which animations and body/clothing/armor models to use for the player character.
Head: Could contain either sliders for specific face parts or preset face shapes. Perhaps both?
Hair: Contains a selection of basic hairstyles. Five options per gender would be a good starting amount.
Clothes: Contains clothing subcategories for Body, Legs, and Feet. Five options per subcategory would be a good starting amount.
Voice: Contains voicetypes for the player character to use when grunting, commenting on things, etc. Could have four voicetypes, two masculine voices and two feminine voices. Perhaps have a pitch slider for more customization?

Color: Every category (except voice) and subcategory could have a selection of colors for further customization. This could be either a selection of predefined colors or a color wheel. Perhaps both?
Saved Teens: If the player has made a custom teen, they can choose to save it for later use when starting a new game or when respawning.
Gender Filter: Every category could have a checkbox to show/hide feminine/masculine options.

Future Expansions: As the game is worked on, the options available for custom teens could be increased via either content updates or DLC.

I think all of your suggestions here are really good. I imagine that they will eventually either release a 5th (non-customizable) character or enable some customization features like you described. The reality is that there is a LOT of programming work involved just to implement the ideas you suggested. So it may come, but may take a while to get there. 

This is the one feature I was a bit bummed out about when I got into the game not long ago.

I like the 4 kids, but none of them are "me".  I understand that for lore/story's sake, maybe these 4 kids were chosen for a reason, or it can ONLY be these 4 kids, etc...  stuff we may not know about yet.

But yeah, I wish you could create your own avatar, even with the simplest of features/options. A huge +1 for this!