Canon Character Fanon

Exactly what it says on the label.

• Big Mac and Sugar Belle's colt is named "Sweet McIntosh," Sweet Mac for short (after a real-world multipurpose apple cultivar that nicely invokes both parents' names). His mane would be as exuberant as his mom's if he didn't keep it cut fairly short.
• Li'l Cheese is formally "Quiche Slice." He was actually designated a filly at birth and named "Ricotta Pie," but figured out pretty early on that he wasn't a filly and the name didn't fit. Thanks to having supportive parents and aunties (and who his mom and some of his aunties are), he hasn't gotten much guff for it...although it's kind of agreed-upon that he's the only one who gets to make "real colts don't like quiche" jokes (because he, of course, is the real colt who willingly took it as a name). The nickname "Li'l Cheese," however, has been around since day one and persisted throughout everything.
• Cup Cake and Carrot Cake retired from Sugarcube Corner after Carrot developed chronic pain from an old back injury (we briefly see him trying to work through it at the spa in canon). Hence, why the twins are running things now.
• Rarity stayed in contact with Kerfuffle. They are regular pen pals, occasional business collaborators, and possibly at least friends with benefits when they actually cross paths.
• Tempest Shadow occasionally lets somecreature call her "Fizzy." If she likes them. Her acuity mark, never seen in canon, likely evokes both fireworks and berry brambles.
• The spacey gray pegasus mailmare's name is Ditzy-Doo (it was originally Bubbles, but she changed it to remind herself to be less scatterbrained), although she's got a few nicknames (also mostly self-inflicted). She has strabismus (and resultant issues with her eyesight), balance issues, and memory trouble due to a head injury sustained when she was a filly.
• The "Love Yourself mare" (whose name is not "Whoa Nelly") has a growth disorder and an acuity for boosting self-esteem. Rarity eventually custom-tailored a dress for her.

I can't even begin to speculate as to what's going on in the Crystal Empire. Flurry Heart clearly came into her own rather spectacularly at some point between "The Ending of the End" and "The Last Problem," but that's about all I've got.