…how much do you think the population will rise?
By looking at this link you can see that Fable 3, which is free right now, is 8th on the Live Activity chart. My guess is that with Halo 3 being free, there could be a big increase in population similar to Fable 3. Could this lead to a resurgence of sorts for Halo 3?
If there are no DLCs included, then only playlists that will see injection of new players are Ranked and Social Team Slayer. Playlists that could use repopulation (like BTB or Team Mythic or Rumble Pit or Ranked playlists) are all DLC-required.
However I hope there will be people who will comeback to Halo 3 because of hype.