Mirkwood

So, lets get one major thing out of the way here in regards to the potential threats to Mirkwood. It is highly unlikely that the Lonely Mountain would be one such problem. First, in the reworking of the plot I will get into elsewhere, the dwarves would have been saved by instead of abandoned by the Mirkwood Elves, which would cut out a lot of the mistrust between the two. Plus, Thranduil was willing to back down in the canon narrative due to an unwillingness to fight a war over gold. Meanwhile, the humanitarian aid to a mutual ally (Dale) would moderate any hostility. Plus, both Mirkwood and the Lonely Mountain have friendly political relations and long standing profitable trade links with Dale. Meaning both sides would trust them and Dale would have an incentive to be an honest be a solid third party mediator between the two. Not to say that they would not have disagreements, but that would probably not boil over into full conflict. Both Dale and the Lonely Mountains would be useful allies as well - major military powers located outside an isolationist power with a weak military are great for buck passing.   First, the Southrons and Corsiars can't reach them even if they wanted to. Gondor and Rohan lack enough military power or incentives to attack. Gondor is too far away. Isengard has several military powers between itself and Mirkwood, plus bigger fish to fry. Not to mention that the section of Mirkwood closest to it is occupied by its allies, so war would not be needed to get the wood located in the south. Isengard would be a long term threat, but Mirkwood would have likely been wiped out long before Isengard becomes an issue.   Essentially, this leaves the Easterlings and Mordor. The result is a relatively simple threat matrix that makes military planning so much easier. The Easterlings are close enough to strike, as we see in the Battle of Dale. Mordor has Dol Guldur literally inside the boundaries of Mirkwood. Easterlings would be a threat both in terms of quality and quantity. Mordor has its trolls and Wraiths based in Dol Guldur that are of solid quality - but its main power is its huge numbers. Even with the many fronts Sauron wants to fight on at the same time, enough forces could be marshaled to make life interesting for Mirkwood. Geography and troop numbers means both have the option to attack.   Only Mordor has an inbuilt precedents and clear motive for war. The forests of Mirkwood would not be clear areas to expand into to secure the food production so desired by Easterlings. So I doubt they would go to war against Mirkwood on their own initiative. But glory in battle under the command of their god means they could easily be pushed into an assault. Mordor would want the destruction of the elves, while the Easterlings would want the territory more than anything. Thus those would be the main fixed priorities they threaten. But they would probably not seek to destabilize from within Mirkwood. But Mordor's coalition is pretty blunt in the fact it directly threatens all three elven variable priorities.   I think the Easterlings would be regional and short term. Yeah, they would need to be dealt with, but they would not threaten Mirkwood in the grand scheme of things. Remove Mordor from the picture and the Easterlings would be removed too. After all, there is no fundamental conflict of vital interests that makes war unavoidable. That is not true for Mordor. The only thing keeping Mordor from wiping out Mirkwood is other targets Sauron prioritizes higher. Thus, the Easterlings would remain steady for the conflict then begin to recede. Meanwhile, Mordor will continue to expand as a threat until its defeat. Both forces would require a high commitment of military resources to combat effectively. Mordor Naturally be the main global, multinational threat while Mirkwood would probably see the Easterlings as a more regional level threat but one that still would need a multinational coalition.   But Mirkwood does have one threat that is unique in Middle Earth. Giant Spiders. Think if Shelob would be an every day occurence in and around where you live. This would be a quite lethal threat to the Mirkwood elves - but unconventional in the sense that they are hardly a traditional military force. They are also sub national in that they are not directed by an outside power for subversive purposes or known apparently for cross border attacks. For example, the evil armies in the Battle of Five Armies had giant worms, wolves and bats. But no spiders. But they do not seem to be organized enough to represent a major military or political threat internally. But the loss of skilled craftsmen due to spiders killing elves and the decline in trade due to the spiders make them and economic threat.