Well it's certainly not any more of a leap than a Mage Wars dungeon crawler (I am quite curious about how that is going to end up working).
Truth be told, I don't think a Grand Strategy variant would be all that hard to implement. Mage Wars already has a simple built-in economy model (mana) and positioning of units. Additionally, Domination added variable terrain and control points (as well as a 2nd economy resource with V'Tar). There's only a couple things that would need to be changed / added:
1. Larger Scale - you would need a LOT more tiles to approach something like a real Grand Strategy game (we're talking 50 plus). Plus a very, very large table. You'd also need to use more cards (not a problem for people who have bought the expansions) and implement limits on unit stacking.
2. Changed Terrain - it would also be far more fitting for a Grand Strategy game to take place on a country or continent wide scale. This would require changing the artwork on the tiles to represent cities/mountains/fields/hills/etc. and making ALL OF THEM possess unique terrain (no blanks).
3. Political System - especially when dealing with more than 2 players, a Grand Strategy pretty much requires some form of political system.
4. Leaders (Mages) as Secondary - most Grand Strategy consider rulers/generals/etc. as secondary to cities and nations. This would necessitate changing the victory conditions away from killing the enemy Mage (at best, it should slow the enemy down or provide an inconvenience), as well as reducing the overall power and importance of Mages (cities cast most spells instead, etc.). Domination did do this to an extent, but it would have to be taken further.
That being said, I cannot even imagine how much time a single game would take. The vast majority of people wouldn't have the patience for it (as sad as that makes me).