Perhaps custom allocators could be interesting for BTreeMap and BTreeSet? I am not familiar with their internal implementation, but as I understand it those should allocate fixed-size blocks, which would benefit from a specialized fixed-size allocator.

Sure, in principle all of the standard library collections and indeed most 3rd party collections which allocate could benefit from providing a good Allocator API.

If there's a particular 3rd party collection you know would benefit, you could ask them, or write a PR to make use of Allocator. The standard library collections of course know people are interested, you can think of this as an MVP.

I just picked a few I personally want to see.