In the diplomacy screen you can see what tech have been researched by your enemies. There's no point in spying if a windows can already give you everything you need to know (you even know their diplomacy skill !!!)

Why not giving some rough estimation of the power of your enemy ? Instead of "Diplomacy : 0" you would have "Weak diplomacy" or the word diplomacy written in a meaningful color (red for weak, green for strong, or something like that).

Moreover, invisible units should be able to travel through other kingdom without declaring war (and letting you spying them). But if a unit with "true seeing" catch you, then you have a casus belli.

What are your thougts about intelligence ? Should we find something in the lines of Medieval total war 2 ? Or something like in civ4 ? Something else ?

EDIT: I forgot to speak about the importance of intel. As Sun Tzu said : "All warfare is based on deception. Hence, when we are able to attack, we must seem unable; when using our forces, we must appear inactive; when we are near, we must make the enemy believe we are far away; when far away, we must make him believe we are near." or "It is said that if you know your enemies and know yourself, you will not be imperiled in a hundred battles; if you do not know your enemies but do know yourself, you will win one and lose one; if you do not know your enemies nor yourself, you will be imperiled in every single battle."

Nuff' said.