Improve builds

[vernal niche]

I guess it would be a great thing to have any system to specialize warriors, birds and trolls into any elemental version, as we do for ballistas.
Maybe a late building that offers a sacrificial altar on the barracks, nest and troll cave, where we can spend either rat nails (poison), wax(fire) or snowball(frost) on a passive tree to specialize these units in one (or multiple ?) elemental damage.

[silent hull]

assumption is there will be many, many more bosses and drops in the future, and majority of specialization will be on the items advisors use (like current warrior poison spec)