A checklist feature in the Description field would be a massive improvement. For developers, ticking off functional requirements as they work is crucial for staying on target. For high-level tracking, keeping everything consolidated to one Task would vastly simplify things (e.g., prep for client meeting: 
write agenda / schedule Zoom call / confirm slide deck / 
invite CEO). Subtasks are easy to lose and include too much functionality (e.g., its own separate due date and description field) for quick ticking. Preferably, ticking a checkbox would strikethrough the copy on that line; additionally, you could make multiple layers in a checklist, just like the Bullet or Numbered lists that exist today.
The text editor in Description is so powerful, yet a basic checklist is glaringly absent. Google Docs and Google Keep have great examples of this functionality.
Alternatively, checklists could be an optional separate component added only to cards that need them (looking at you, Trello). In this approach, ideally the user could expand the checklist in Board view or hide it if not relevant.