So we know what the tactics, lore and neutral player side quests do. Any guesses as to what the remaining two, provided we get these cards at all, might do?

The spirit side quest might be as simple as lowering each players threat by a certain amount once explored or could do something surprising and unique. Not sure what this might be.. maybe condition removal? We could certainly use one more great card that does this, we did JUST get Athelas and Elrond ally fairly recently though..

The leadership side quest I think may just allow each player to put an ally card into play for free or maybe even an attachment for free? This could potentially be very strong though so if this was the case maybe it will have more quest points than the 4 that the neutral and lore side quests have? (The tactics one has as it also has the battle keyword as well as its effect which can be very powerful) Maybe the leadership side quest will allow each player to draw a card and ready a character when explored. Any ideas?

Well, I don't know if this is a prediction as much as a hope, but I would love to have a Spirit sidequest that helped get rid of locations in the staging area so there is a legit alternative to Northern Tracker. The card could read something like this:

Explore Surroundings: Before progress can be placed on this quest card it must be placed on locations in the staging area first.

1 quest point, 1 VP

The only potential problem I see is that I think this would get around locations that are immune to player card effects, because the progress is not being placed by any card effect, but by normal questing. Still, I can hope.

Side Quest: Each player adds 1 resource to each hero he controls.