Scout heroes need something what can enhance their scouting purpose. Armor which slows and transforms them into warriors doesn't fit to scouting role, cause they're still weak even after armor upgrade. But if we want to keep armor, it can be implemented via levelling and available at level 10.
Level 1: Rumil and Orophin - Both heroes share combat experience, reveal hidden enemy units nearby, and can use their skills simultaneously while both are selected. At level 10 put on mighty elven armor that highly enhances their defense but lowers their speed. (Passive ability).
I removed attack bonus for Celeborn, cause armor should add only bonus for defense - logically.
Thranduil now gains armor normally while levelling. Because he would accept only some gems, enchanted silver gem from Noldor sorceress was obvious choice.
For Grimbeorn is natural Axe of Doriath which proposes Necro, although I added only splash damage and not attack bonus. We know that even Gimli, such elven hater, accepted gift from lady and protected three golden hairs like the greatest treasure in the world, so why not give the gift option to Grimbeorn as well. Especially when he fits to forest and protects nature like elven woods.
And finally, for trees is extremely vital water and water is dominant element of Lothlorien, so blessed water from mystical stream fits very well for agile but vulnerable Quickbeam.
And shepherd of the forest? We completely forgot that Treebeard would definitely accept the gift from the lady, he has great respect to her:
Then Treebeard said farewell to each of them in turn, and he bowed three times slowly and with great reverence to Celeborn and Galadriel. "It is long, long since we met by stock or by stone, A vanimar vanimálion nostari!" he said. "It is sad that we should meet only thus at the ending. For the world is changing: I feel it in the water, I feel it in the earth, and I smell it in the air. I do not think we shall meet again.
And Celeborn said: "I do not know, Eldest." But Galadriel said: "Not in Middle-earth, nor until the lands that lie under the wave are lifted up again. Then in the willow-meads of Tasarinan we may meet in the Spring. Farewell!"
So Sam's former gift that spawns huorn instead of Mallorn sounds like great idea.
Overall it's bad idea to separate Grimbeorn and Ents when they fight for the lady in Edain Lothlorien's faction. From gameplay perspective, it would also add more diversity - player would choose how to play with Galadriel's skillset pattern from which in total three heroes are currently excluded. Grimbeorn and Ents are now disadvantaged in comparison with other heroes.
By the way, if Angmar's Witch king similar ability affects all heroes, so why not Galadriel?
Level 5: Might of the Witch-King - The might of the Witch-King will be bestowed to a desired hero with a specific effect:
Zaphragor: For a short time, the area around him will suffer from multiple explosions every few seconds.
Mornamarth: For a short time, Men of Carn Dum in the radius of "Lust for Power" have 30% more armor.
Gulzár: For a short time, Gulzar's spells reload twice as fast
Dûrmarth: For a short time, "Blade of the Enforcer" deals double damage
Drauglin: For a short time, Drauglin is transformed into a wolf beast with more damage and health. However he must transform back into a human before the timer expires or he will die.
Karsh: For a short time, Karsh is healed by 75% of his dealt damage
Hwaldar: For a short time, Hwaldar gains better stats.
Helegwen: For a short time, Helegwen's arrows pierce through enemies.
Left click on a hero of Angmar.
To conclude, now all heroes can receive unique gifts, which makes Galadriel's ability more useful and really unique!