It would be nice to be able to add a tunable to TTL Indexes to change the priority of the cleanup thread.
For systems with High Loads, slightly higher priorities will prevent heavy backups of the cleanups. And lower access systems might require a lower priority than what it is set at.
It is possible for heavy write loads to fall days behind in terms of your TTL cleanup and it is possible for the TTL to effect performance slightly. Being able to have more fine grained control of it depending on what you want would be great.