A nation the AI could use *well*

[Twan]

The immobile mages + huge gem income wouldn't result in a balanced nation that players could make use of though.

Why not ? Imagine a nation with an immobile capitol mage with the level to cast an offensive ritual (say F3 for fires from afar or A3 for seeking arrow) and a 15 one color only gems site, but handicaped in both magical diversity (2 elemental paths, the hardest to boost, fire and air) and quality of mages able to move (say F1A1 + 100% FA + 10% FA ; so a mage can always use fire or air evocations, sometimes the two, but never forge boosters).

Even if troops are good and all national leaders get some supplybonus to help the AI, it would be a rather weak nation for a player (the need to empower mages compensate the huge income, and the lack of magical diversity the research advantage fire gems can offer). But as the AI doesn't forge boosters even when possible, and like to waste gems on offensive rituals and empowerements even when it doesn't have this increased income, it's all good for its playstyle.