I'd love to see another hero version of Elladan and Elrohir (without abilities explicitly tied to each other), just because it would open up deckbuilding with the existing two heroes.
I would love to see cards that make these heroes more playable. I think we would've seen them already if they were being planned though. Weren't they at helms deep? Did they come with Aragorn after helms deep? I can't remember.
If they do release allies I would hope they have a search and find effect like Fili and Kili.
To be clear, I'm not particularly expecting Elladan/Elrohir allies in the final saga expansion, and certainly not new hero versions. They came with the Grey Company, bear counsel from Elrond, and help out with healing in Minas Tirith, but aren't really major characters. But I would like to see new versions of Elladan/Elrohir in both ally and hero form, to make the current Elladan and Elrohir not so joined at the hip.
For that reason, I wouldn't really want to see a Kili/Fili effect for the ally versions, since it would be a wasted effect for a Elladan ally in an Elrohir hero deck. I would like to see some sort of ability that keys on the other being active, so that the ally versions have synergy with each other as well with the hero versions.