I have to vote on this and then get back to work finagling monday to do what I need it to do!
Thank you @timlittletech for the checklist idea - it might help for some use-cases, but not very reportable, and would be hard to automate with. (I’m thinking of progress indicators here like 7/10 checkboxes means this subitem is 70% done.)
The way subitems seem to be handled, IMHO, needs a lot of thought. (Only three automations come up as as a suggestion, and you can’t build your own having subitems are just some of the examples.)
Linking the boards isn’t a practical option in the long run either. It becomes very hard to build a template to roll-out a new project where you have to create a ton of links between boards just to get them to work, it almost defeats the purpose of having the templates, other than you have consistent shells each time.
There are many times that you need to have sub-subitems. If anyone has managed staff before, you can most likely name two past (or present) employees that are on the opposite ends: one you can tell “put on your shoes,” and the other that needs “place left foot into left shoe, tighten laces on left shoe, tie left shoe…” The task of putting on your shoes is already part of the task “prepare to go outside,” so where do you put those detailed instructions? (I’m sorry if you live in an area where you just put on sandals and head out to the beach - I’m jealous and won’t help you! 
)
This may be a simplistic example, but there are many real-world applications that require steps deeper than just a subitem or subtask. The example used in PCs for example, could also be extended. The days of just a monitor, terminal and keyboard are long gone. A PC has several components to it, and can have several accessories attached. This would end up creating spaghetti-lookups, and much like spaghetti-code, is not ideal. (Now I’m hungry and have to get back to work!)
tldr; We need more than just the one subitem level, but could probably adapt to monday much easier if you could do more with the subitems, or at least had sub-subitems.