Goals, design,and roadmap

I need this project to get somewhere useful fast. For that, I'm going to review its design and plan a roadmap that has an actual chance of letting me finish my PhD.

Let's try a high concept:

Grimoire: Ars Belica is about being a spellcaster during war. Your power is worth ten or a hundred times that of a single soldier. Your surgical use of magic makes the difference between life and death.

You fight against the hordes of horror. To defeat them, you must destroy their portals. However, their portals lie deep into the chambers of death. To prevent them from crumbling civilization's last walls, you must invade them yourself.

Sounds decent. Now, what are the most important aspects of it? What parts of the game need to work for this aesthetic to manifest itself?

1. Creatures. Lots of moving, battling, tactical creatures.
2. The walls (any breach means defeat)
3. The portals (must destroy all)
4. Fucking magic
5. A map

To be continued...