Any chance there is going to be a Legio Custodes force added to the list?
They're not in the near pipeline as far as I'm concerned though we could look to get some draft rules out by the end of the year.
Priority at the moment is the next Legion update, an update to Mechanicum and Solaris, then some proper work on the specific legions.
Longer term may be an "allies" list covering off on imperial navy (allowing you to use a range of flyer models regardless of army), Imperrial/traitor militia forces and forces like the Custodes.
-these would be allowable under the 33% allies limitation, though it will take some work to figure out how they effect strategy and balance. IMO, these forces work best as allies rather than armies in their own right.
The end result would be you could lead your Imperial fists with attached Custodes detachment againts your mates Word Bearers and their horde of cheap cultists... in theory...