Toilet sits directly over sewer return -- what sort of venting?
There are those that still maintain that a toilet needs a vent.
Take a look at this question here for one such opinion: Do I need to add a vent when moving a toilet drain across the bathroom?
However, the IRC has an exception for toilets (water closets).
From the 2009 International Residential Building Code

SECTION P3105 FIXTURE VENTS
P3105.1 Distance of trap from vent. Each fixture trap shall have a protecting vent located so that the slope and the developed length in the fixture drain from the trap weir to the vent fitting are within the requirements set forth in Table P3105.1.
Exception: The developed length of the fixture drain from the trap weir to the vent fitting for self-siphoning fixtures, such as water closets, shall not be limited.

So, the IRC is basically saying with a toilet (WC) you can run as far as you need to horizontally to get to the nearest stack. It would seem then that you could make a new stack and drop into the sewer line using a wye and an eighth. If all the rest of your plumbing is properly vented you should not have any issues.
Now, if you are making a powder room you will have a sink in the room also that needs to be drained and that definitely needs to be vented properly. (up to the attic and tie into the rest of the vent system) And with that sink drain line you could wet vent the toilet line up to the dry portion of the vent.
Good luck!