diff --git a/TODO b/TODO
index a16bb74ea8..90da24dd67 100644
--- a/TODO
+++ b/TODO
@@ -133,7 +133,8 @@ Features:
 * systemd-nspawn: something like --volatile= but that isn't volatile, but
   stores the data in some separate dir on disk. Usecase: keep always up-to-date
   DDIs of some OS in your home dir, but combine its /usr/ with a locally
-  maintained root fs in a regular dir to maintain local state.
+  maintained root fs in a regular dir to maintain local state. (idea: call it
+  --mutable= and take dir or DDI and merge in)
 
 * maybe rework systemd-modules-load to be a generator that just instantiates
   modprobe@.service a bunch of times