Druid talent Nature's Grace would be a good addition.

Talent https://www.wowhead.com/wotlk/spell=61346/natures-grace
Buff https://www.wowhead.com/wotlk/spell=16886/natures-grace

It's a bit atypical because it gives a buff for 3 seconds, rather than an effect "on next cast". However, due to the short duration, the buff will most likely help one spell cast (or two at most) so it makes sense to treat it as a "next cast" ability.

The chosen location would be "Top", which is not used by balance druids yet, with the serendipity texture because "the kind of moon-flashy-proc-thingy kinda fits the balance aspect", as mentioned here.

Implemented in #101.