Tassadar has always been represented by a high templar model. He has always been the iconic templar character and the two iconic templar abilities are Storm and Archon.
If they did a bit of research on Templar they will find that Shields, Forcefield and Phase shift were considered for the High Templar as possible abilities.
Oracle seems like a natural extension of Hallucination in it's scouting potential. It also kind of goes with a powerful Templar who is so psionically attuned he can see everything.
With Hallucination beeing moved from the High Templar to the sentry in starcraft 2 it's not that much of a stretch to assume that High Templar are capable of using Sentry based abilities.
As far as Tassadar being an assassin that doesn't make sense and doesn't fit with the fantasy of the High templar. Apart from storm and archon every other ability that has been assaigned to the unit in a public capacity has been support based. (Hallucination, Plasma Surge, phase shift, force field, and Time Rift)