Things has by far the nicest UI and frictionless interaction design, but I’m more and more turned off by it. 
- no end-to-end encryption or iCloud sync option
- location based reminders are a must (it sucks to use a different tool just for that)
- any kind of timeblock/calendar view would be desirable (see Notion or TickTick)
- there still is no way to save searches/views that logically combine areas, tags and other conditions (similar to Omnifocus’ perspectives, no widgets do not suffice)
- generally I would love to have toggleble states for a todo following the GTD methodology, such as active/waiting/delegated
and while we are at it:
- allow Markdown formatting in the notes area, especially to abbreviate links for example of those long url-scheme ones from a Bear note or Airmail mail.