A Simple suggestion (i think): make Radiant Destruction an Ultimate Skill, and turn Nova into a normal skill.
From a Utility point of view you gain a lot more from Nova compared to Radiant Destruction. Both the damage for offense, as well as the Damage reduction for support (and the Synergy for group play) are fairly useful, more so than the damage from Radiant Destruction.
Both the basic skill as well as the Solar Prison Morph could cost magicka, while the Solar Disturbance morph could become a Stamina version.
For the damage and cost ratio, they could make it cost a lot and keep the damage as it is (Should be around 1.5-2X the current Radiant Destruction cost with the current damage), or reduce the damage a bit.
Making Radiant Destruction an Ultimate would make it more of an actual finisher and less of a "keep-spamming-it-because-the-enemy-might-drop-below-a-certain-percentage-of-health" skill.
Increasing the damage a bit shouldn't be a problem, as you can no longer spam it as an ultimate.
Damage equal to Soul Assault (because of the duration), together with the execute could be an option, or DECREASE the damage quite a bit (about half it is now), but make the execute phase deal significantly more damage (up towards 900% bonus damage).
Second option would set Radiant apart from Soul Assault a bit more.
Mending-The-Wounded, Aldmeri Dominion, Templar.