This is a big-picture analysis, not speculation or desired propositions, of missing themes with our current sets of elite specializations. These missing themes may be included in the third expansion if this expansion introduces a third set of elite specializations. It seems like the themes of current elite specializations focus and expand upon existing themes within their respective core professions, and then the elite specialization themes add a unique twist on top of the focused themes. These twist occasionally create the secondary profession flavor from GW1.
In this analysis I will not imagine new themes that do not already exist within core professions, but thematic twists in elite specializations occasionally create new themes. For example, the Druid focuses on the nature magic from the Ranger, and the Druid also adds a twist with astral-flavored abilities. Astral abilities do not significantly exist in the core professions. I will not be stretching my imagination to predict added-twist themes like the astral theme, but please consider, with twists, that elite specialization themes are not limited to the narrow thematic gaps I will identify below.
Additionally, I will not analyze missing play styles (ranged, brawler, skirmisher, etc.), roles (damage, support, utility, etc.), or weapons with each profession and their respective elite specializations. This analysis is focused purely on themes and flavor.
Revenant: This profession is the profession of all professions. Elite specializations do not expand upon existing Revenant themes. Elite specializations add on to existing Revenant themes. Because of this, I cannot identify missing themes for this wild card profession.