Some ideas: raiding, seiging, spell AI and more..

[baruk]

Graeme Dice said:

A turn represents an entire month. I'm not sure why stepping through a gateway would make you more tired than a month long march into enemy territory.

<Thinks on feet> According to my dominions medical textbook of dimensional diseases, Planar Sickness Lasts exactly one month of game time. Fancy that!
Who knows what horrors lurk between worlds? It would give me the heebie-jeebies, at any rate.

baruk said:Arranging things so that your mage fights several battles in a turn may be tricky, even with potentially 11 battle rounds a turn.

Graeme Dice said:You can expect to see at least two battles a turn on average for major armies. More if there are multiple opponents all attacking you at once.

An army will still have to be cut off from its gem supply when on offensive maneuvers eg. when seiging castles, or taking unlabbed enemy provinces. This means there is some scope for turn to turn gem attrition.
I think my proposed change wouldn't be too unbalanced. The potential horror could be a pretender SC with many gems, using them to cast battlefield spells, annhilating a succession of small armies in the same turn. A willing or unwary set of opponents and some luck would still be required. I would guess that it'd be difficult for an attacker to actively arrange multiple battles. It doesn't worry me because people can and will adapt to the new tactical environment.