You're essentially asking "What system folders can I load into RAM rather than save on the disk".
The short and vague answer is that "You can load any folders into RAM by making them 'tmpfs' mount points in
/etc/fstab, if and only if the folders contain temporary files or items that do not need to survive reboots", and, "We cannot answer this question with any specificity because there is no bounds on the scope of the question."
The longer, but equally as vague, answer is "What can or cannot be loaded into RAM cannot be described in the scope of a short answer here". Different systems and software handle caching of information, or storage of temporary files, slightly differently, and none of them are usually designed to utilize a
ramdisk to store those files, as some of them need their caches to survive reboots.
For all intents and purposes, the only 'safe' directory for being loaded into RAM this way is
/tmp, as the contents of that directory are extrememly regularly erased, by reboots. It should be kept in mind, though, that if you want to do this with
/tmp you will likely need to give it a larger amount of ramdisk space (such as
1GB) which will detract from your available memory on the system.
TL;DR: Within the scope of this site,
/tmp is one of the few items that can be sanely put into a RAM disk /
tmpfs space. We cannot comment on whether any other system directories can be loaded into
tmpfs, as we do not know the needs of your system, the needs of the software you need, nor what your ultimate goal is with such an endeavor.