Thank you all.
I think I have enough inputs/ideas about Bardings and Woodmen population, Beorn's personality and the Necromancer's behavior.
Now, put aside these background concerns, I'm ready for the main course of an adventure: the enemy.

Now that the Necromancer is gone I foresee that most of my pc's efforts would be about reclaiming the Old Forest Road for the free peoples.
As major, recurring menace I rule out Orcs, for three reasons:
-There is no major Orc hold in the zone
-The Orcs now are disorganized and demoralized. They will become a major enemy after 2951.
-Frankly, everyone playing in M-e expect orcs and I want something different.

So I think the main opponents on the Old Forest Road would be the Spiders.
Because it's their territory, because Tolkien's Spiders are uncommonly intelligent and because Spiders are scary.

I was thinking about them. Usually giant insects/arachnids are a secondary opponent in rpg. Dangerous maybe, but lacking rationale, motives and depth. They exists, but don't want nothing more than eat/be left alone.
I want to give these Spiders an organization, some ambition, some craving. But what?
Then I read Eluadin's post about the Dwarfen Rings (LINK). Something clicked in me.
There is an ancient Spider in the depths of Mirkwood, cunning and cruel beyon measure, and feared even among his kin. And he got one of the lost Rings of the Dwarves.

The ultimate secret of my campaign, and the final enemy of my pcs.

What do you think?

Okay ... I've had some time that I could sit down and consider writing this up.

Dindair

ATTRIBUTE LEVEL 8
ENDURANCE 55
HATE 8
PARRY 8
ARMOUR 3d *

SKILLS
Personality, 2
Movement, 3
Perception, 3
Survival, 3
Custom, 2
Vocation, 2

WEAPON SKILLS
Javelins of Shadow ** 3
Damage 7, Edge Eye of Sauron, Injury 14, CS poison

SPECIAL ABILITIES
Dreadful Spell (Shadows and Phantoms)
Reduce the Dindair’s Hate point score by one to gain one of the following effects: 1) reduce the Parry rating of a hero to the basic combat TN of their chosen stance; 2) gain a +2d Movement bonus to Stealth; 3) create completely illusionary persons within 1000’ feet (TN18 Awareness test to realize it is not real); and, 4) create an illusory landscape up to 8000’ feet in radius (TN18 Awareness test to realize it is not real).
Dreadful Spell (Veiling Shadow)
Reduce the Dindair’s Hate point score by one to call down a cloud of shadow that covers an area with a radius of 80’.
Hatred (Elves)
When the creature is confronted by the object of its hate, all its Weapon skills and Attack forms are considered to be favoured.
In the Shadow of Others
The Dindair may imitate a single quality (attribute, skill, weapon, specialty, special ability, cultural blessing, physical appearance, virtue, etc.) of anyone in its shadow. The Dindair employs that quality with exactly the same magnitude as would the imitated person, including bonuses. However, the imitated person gains a +1d bonus to all tasks and tests against the Dindair.
Javelins of Shadow **
Except when in direct light or absolute darkness, the Dindair can form javelins from the shadows. The Called Shot is a magical poison. A poisoned character is paralyzed and falls to the ground after one round, in addition to being considered to be Wounded. The effects of the poison wear off after one full day.
Shadow Armour *
The Dindair is cloaked in shadows that provide it with the indicated armour, as well as protection against direct sunlight. If exposed to sunlight, 1d of armour burns away each minute until gone. After that, the Dindair sustains the effects of its Susceptibility to Sunlight normally. Each minute spent in shadow permits the Dindair to reweave 1d of its lost armour.
Shadow Guise
The Dindair can alter its appearance slightly so that it may pass unnoticed and unmarked among other folk (TN18 Awareness test to recognize the deception).
Shadow of Fear (TN 16)
Reduce the Dindair’s Hate point score by one to force all companions to make a Fear test.
Shadow Veil
The Dindair is shielded from all scrying attempts.
Susceptible to Sunlight
The Dindair loses one point of Hate at the end of the each round of combat fought while exposed to direct sunlight.

Thoughts? Suggestions?

Corvo,

re:Necromancer
The Necromancer does not need to be completely removed, just placed farther back in the hierarchy of enemies. Three or four levels farther up, at least, should be sufficient - provided he is out of the character's reach.

Sauron is hiding behind the Necromancer facade and does not want to be correctly identified. Sauron is a master of subterfuge, manipulation, and scheming who is willing to work directly and indirectly to accomplish his goals. Even though he left Dol Guldur, the Necromancer did not abandon the site.


re: Orcs
Keep the orcs, after all Beorn specifically states that goblins (orcs) are using the Forest Road. So they cannot be completely removed. They just are not the focus of your campaign.

As for the presence of orcs in general, while no major orc outpost is shown on any official TOR or Tolkien map, the Necromancer's evil presence, and hidden influence, pervades all of Mirkwood. Thus the orcs are here because evil, in general, is in Mirkwood. They're not here to serve the Necromancer specifically (even though that really is the case). So scatter orc clans around like they were Woodsmen villages, in little clumps (population 5-50).


re: Elven Ruins
Keep in mind that Mirkwood was once Greenwood the Great, a forest settled by Silvan (Wood) elves. Elven settlements and other signs of Elvish occupation would be scattered all through the forest. Most of which would have fallen into ruin.

re: Spiders
The use of spiders as the main protagonist could be quite interesting depending on how much attention you give this idea. I would suggest reading some kind of nature guide and examining different types of Arachnids to come up with additional spider variants (body type, attacks, size, etc).

I can only conceive of one scenario in which Sauron would allow Giant Spiders, which are demonic in origin and thus potentially immortal, to acquire and keep a Ring of any power. It must be an attempt by him to gain control / mastery over the spiders. This seems to be the only way that Sauron ever utilized the Rings. Also, he would never allow the spiders to gain a Greater Ring of Power - far too strong after Ungoliant's treachery. If Sauron was trying to use a Ring on the spiders it would be a very weak and seriously flawed Lesser Ring.

The only thing the Ring can do for the Spiders, really, is to act as a refrigerator. Allowing prey to be preserved and stored for a much longer period of time. Freed from constant hunger, perhaps the Spiders are able to start acting in a more organized manner. They are starting to gather and domesticate living creatures (read: animals, people, orcs, etc) for later consumption.

Garn, thank you for the extensive answers.

About Orcs: I agree. As I wrote, they are not the "main recurring enemy" for the time being.

About the ring and Sauron. I think Sauron didn't and cannot allow anything. If Gollum found and kept The One for centuries, it's believable that a Great Spider ended in possession of one of the Seven.
And Sauron spent centuries searching the Anduin for The One, while it was in Moria, at his doorstep. I doubt his capability to locate one of the Seven.

About the ring and the spider. Your idea (the refrigerator) is possible, but not fun
In Eluadin's post I read about greed, and avarice, and enhancing someone's natural weaknesses. This is what I'm shooting for.
Everybody expects the spiders as nasty but simple creatures. I want to surprise my players. I want a self-conscious, malicious spider, with dark thoughts and ambitions.
The Wargs got The Werewolf. I'm envisioning something similar for the arachnids.