There's a big red disclaimer on this page that says the feature list is a bit wishful, and the latest version is only 0.1.0. The docs (man pages, start with man aerc) only document implemented features, you may want to start there.
Several people have expressed interest in working on this, I expect we'll see it for 0.2.0.
As far as configurability is concerned, yeah, the plan is to continue making it more customizable and configurable. Stuff like filters already afford you a lot of options today. At some point I might also be interested in adding a fifo you can use to feed commands into it from a script, too, for turing-complete customization.
I was looking at the git page and wondering where I could see discussion on features/bugs etc, I did not find it and was a bit frustrated.
I found out reading this comment that not only it has a todo page but it works great! Not sure if it's a design choice but perhaps making the resources accessible in a single page (e.g. like a project home) would be something worth of consideration.
Here's the ticket for maildir support:
https://todo.sr.ht/~sircmpwn/aerc2/16
Several people have expressed interest in working on this, I expect we'll see it for 0.2.0.
As far as configurability is concerned, yeah, the plan is to continue making it more customizable and configurable. Stuff like filters already afford you a lot of options today. At some point I might also be interested in adding a fifo you can use to feed commands into it from a script, too, for turing-complete customization.