First of all i'd like to say.. please don't flame this about how 'you oh so hate microtransactions'. This is a topic about what types of microtransaction ideas you wouldn't mind (or might even like) to see, not about whether or not you like these transactions. If you hate them all, then I suggest you just don't post here.
*Feel free to critique other people's suggestions, however.
With that out of the way - what would you like to see, or wouldn't mind seeing the devs charging real money for? I don't believe they have said that they would, definitively, use microtransactions (so don't have a heart attack), but if they did.. what would you suggest they charge for?
Here are some of my ideas:
Stronghold relocation: Paying real life money for an instant relocation of your base and army within that base. I would not, however, support this unless there was also a method for moving your base using in-game resources instead of money.
Banner symbols: I wouldn't mind them charging for some particularly awesome banner icons to customize our banners with.
Small, long-term buffs: By this, I mean paying to include a buff that isn't very significant.. but lasts for a long time. (Say a month). This could be like a 1% increase to loot, or a 5% faster troop recruitment time, 10% faster army march speed, etc. Basically, something that is designed to be pleasant, but not enough to give someone a significant advantage. The appeal would be in that these buffs last a long time - thus the 'cumulative' benefit is still significant.
Special unit designs: By this, I mean being able to purchase something that would change the 'model' of a specific type of unit into a cooler-looking version. No statistical or practical benefit, just a change to the look of the unit. For instance: perhaps making a berserker have big spikey armor, etc.
"He uttered the word of curse, crush him with our Orcish iron!"