Introducing a SC4 mod: MySimtropolis @Cyclone Boom and I have decoded the file structure of the MySims.dat file such that we can now create it from scratch with a hex editor. This is used to add additional MySims to the selection window. We have now released the mod which features members of Simtropolis as selectable MySims. (This is an opt-in only creation.)
Note:
Any member who has an established, non-changing avatar on ST can still post here to request inclusion in our mod.
Here's a pic of those who initially agreed (via PM) to be immortalized in the game: ^ Image created by @Cyclone Boom   The initial MySims are coded in the game so new, additional ones are available on page two and after. Until a full second page fills up, clicking the number 2 on the selection window scootches the Sims to the left with the newest ones showing on the lower right. Here's how those look so far:   Update: After we discovered a way to prevent the 21 default MySims being listed, it now looks like this with the optional "Rulez" installed:   In addition to the name, the only two variables which can be set are gender and zodiac sign as follows: Gender: 0x00 = Female
0x01 = Male Zodiac sign: 0x00 = Capricorn
0x01 = Aquarius
0x02 = Pisces
0x03 = Aries
0x04 = Taurus
0x05 = Gemini
0x06 = Cancer
0x07 = Leo
0x08 = Virgo
0x09 = Libra
0x0A = Scorpio
0x0B = Sagittarius
Should you agree to be included we will create the 36x41 pixel game image from your avatar.
(Or you can supply one for us in 24 bit BMP format which we might tweak for positioning and size to fit within the circle.)
  To become a MySim, please submit the following: Name  (As it'll be in the game. Screen name from ST or a derivative like I do.) Gender  (Male/Female) Zodiac Sign  (See list above.) Disclaimer: Due to the nature of the file structure you cannot be removed after opting in.
Here's hoping you'll agree to be added as a Simtropolis MySim. Sign up in a post below.
As a bonus, everyone who does will receive a special badge on their profile. Like so:   Just a note to clarify for everyone...  In reply to everyone who opts-in, prior to release we'll post images of the MySim Selection Panel and also your Sim moved into a home. These preview pics posted are simply to show what each of you look like in the game using vanilla Maxis residentials. The main idea being to ensure you're happy with the avatar as shown, since some require very slight cropping, stretching, or editing to ensure they're displayed properly in view. Once we release the mod on the STEX, you're welcome to place yourselves in whatever architectural style you prefer, wherever you'd like. This could be in a posh suburban estate, a skyscraper tower, somewhere in the middle of nowhere, on an idyllic remote island, or even on another planet! The choice is completely yours and we'd love to see your own MySimtropolis pics. In fact, coinciding with the unveiling of the mod (which won't be too long away), the plan is we'll open a dedicated " Show Us Your MySimtropolis Sims " topic focussed around sharing scenes. Even if you've never or don't usually play with the MySim Mode, this aims to give the community a way to see one other in SC4. With a bit of imagination, perhaps it could even inspire storylines and a few City Journal type posts, or even actual running CJs following the life of your Sim. We think there's plenty of potential.   Additional info in spoiler box:
Just published (20 January, 2019): MySims.dat - The Techy, Hexy Details
  ^ This guide is only for those who might want to add MySims for their own personal use in the game.