Edit extended Family Trees
I'd love it if in manage worlds you can edit family trees, so you can update families that are added per pack/gallery/etc. Many people love the family trees of the main Sim families, like the Goth, Crumplebottom, Bachelor and Caliente families which are connected together throughout the series. So many marriages, cousins, etc.
And allow TBA/placeholders for unknowns and not yet added to the game Sims. Also allow for NPCs we cannot control so they are not left out. This would help with non-canon romancing with extended family from happening.
edit: Last year made templates for the Sims wiki for family trees. And set up the cross-over legacies for the Goth/Crumplebottom/Bachelor/Caliente Family Tree. For example on why we need a more complex family tree system.