[William Cloud Hicklin]

All of Southern Mirkwood was under the control of the Necromancer, which means that having Thranduil come to the aid of Gondor vs the Balchoth pretty much doesn't work, unless they were to go by way of the Carrock and Lorien and come down from the north on the right bank (or swing around by way of Nearer Rhun!) Note that the Eotheod's passage was screened by Galadriel's mist, which implies that Dol Guldur was a very real threat even to swift-moving horsemen following Anduin, much less any Elves trying to move straight through the forest.

But if this is the Jacksonverse, then sure: have Legolas, Tauriel, Haldir, Elrond and Young Aragorn all turn up without explanation. Throw in Gandalf, and Bilbo's great^6 grandfather Pongo. Don't forget to have a dwarf to do pratfalls and belch.