The whole 'underground' idea is well in line with the rest of the game mechanics of TFW, so I think this could be an interesting development.
As we know, factions are 'theme-based' rather than 'lore-based' (for the sole reason that noone decided to develop the lore beyond the bare minimum?), yet 'Burrow' as a trait, or perhaps 'Underground' as an ability, fits better with some factions (Elementals) rather than others (Sylvan). So it wouldn't be surprising to see some factions benefiting more from this than others, with the usual counter-cards in place.
For example, if Sylvan is limited in the kind of Underground 'stuff' they can use, perhaps they can have traits such as 'Observer' where, say, a Thistle Falcon has the ability to detect underground creatures. The ability to Observer can function just like Vision, yet the Falcon might have better/worse Observation than Vision.
And along these lines, you can steal much more from SC2
