Sounds like a good plan
, and can always be tweaked as you go along, to suit not only your own playstyle, but most importantly, the key aims you want to achieve for the role you want your your character to perform.
Evocation sounds like a great way to go, since it nets you the use of offensive spells doing full dmg + one AoE insta-kill via Implosion. Being hybrid, I also tend to invest in 1, max 2 schools, since there are so many bonuses required to make both aspects of a hybrid work (caster + melee).
Having said so, the new spell spec feat @31 made it a lot easier to invest on a secondary school: I'd say it might be worth going necromancy if you decide to go down that route, since it would synergise with neg AoE spellcasting at melee range (with the added benefit of healing you while damaging enemies) and some extra insta-kill spells to add to your rotation. This would involve adding extra complexity to your gear tetris, for sure, but seems a worthy direction to possibly take. The real hard work is making one school work: since many of those bonuses are spell mastery, you'd be a good way already into the investment needed for a second school, anyway.
A further key synergy to consider is the imbue, scaling with spell power. DA is evil damage, while Warpriest fire. I'm sure you have considered this, just thought I'd suggest you go all in one main spellpower, and focus your main rotation of those spells mostly, so you make the most of the synergy of imbue + spell dmg. The issue with fire is that is generally widely resisted by enemies, but would give you a wide range of spells to cast (including the potential for using Dragon Breath: Energy Vortex). Another thought: though there would be significant losses not going Shifter, Tiefling is a potentially desirable option for its fire immunity stripping, if you forsee yourself relying heavily on the fire dmg side of the hybrid aspect.
