Do we have any idea who the L!Hero could be?

obviously, not Alm.

Until I don't find W!Tharja,I don't care,I'm not saving orbs

Idk, all I know is Marth and Hector are coming back.

It's probably colorless so I'm expecting Micaiah, though L!Xander could happen. Probably not colorless but eh, whatever.

Well Eliwood did have the anima affinity in his game.
And Micaiah's affinity and connection to Yune points towards dark.

Finally Seliph and light is an obvious connection

What we know:
1) Colorless is confirmed to be the color for December
2) Eir has been confirmed to be coming back late in the month
3) Mythic Heroes have been confirmed to be appearing on Legendary banners

All the signs are pointing to Eir, but since there can now be multiple Legendary/Mythic Heroes per color in a banner, there's a chance we're getting someone completely new.