A few suggestions:
About Find My:
Please add location history, both for my devices and other people’s (other people should obviously be able to choose whether to share current and past location, or only the current one).
Add option to blur (make it approximative) one’s location based on the time and date, or other triggers (such as shortcuts);
maybe add the possibility to require one’s position to be precise, but let the device owner interrupt such a request (if the owner doesn’t act, it goes through).
About Messages:
Make it revert back to the conversations’ screen after the app has been in the background for some time:
if I launch it after a day it goes back to the chat I was in when I collapsed the app, which means both the chat gets marked as read, and the other person may have the messages marked as read.
Add “relay id” (such as the Hide My Email feature in iCloud), which could be useful for sharing your ID with colleagues, one-time acquaintances without sharing your number or main e-mail.
About iOS:
Make some operations (such as copying files, or sharing them) more background-friendly (I am not sure this is Apple’s fault, still they should enforce iOS to handle such actions with the screen off or the app in background).
Add user-added multiaccounts, or at least let us keep multiple copies of the same app with different data, and let us choose which app communicates with which apps (I don’t like my gmail app to handle both private and enterprise e-mails, and my YouTube app to detect both my private and enterprise accounts; this could be solved by Google, still Apple should let me keep YouTube, GMail, and all other Google Apps’ data apart).