Hi @Christopher_Ayers 
and thanks for taking the time to share your feedback with us!
Subtasks are a vast and dividing topic in the forum, and I believe some of the feedback you’re sharing has already been shared in the past. I’ll try and list below some of them, so you can upvote them. But generally speaking, we ask customers and Ambassadors to keep one request or feedback per thread rather than share a list of feedback. This helps keep the conversation on topic and makes it easier for us to report and follow up on your feedback.
This is happening because there isn’t a built in dependency between subtasks and their parent task. You can create these dependencies manually.
You can upvote Show project on subtask in 'My tasks' view
You can upvote Approvals - Request change should keep the task incomplete
I’m not sure what you exactly mean by this. We recently implemented some updates for subtasks on timeline, you can learn about it in Subtasks are now visible on Timeline!. The main request we received after this launch is to Allow to stack multiple tasks in Timeline on a single line, if this is what you are referring to, you can add your vote to this thread.
You may want to look into Add Subtask Icons in Templates 2.0 