Observation Balloons
I think that the whole aerial unit issue, whether handled by independant units or brigades, should actually be availible from the start (1835) by modelling aerial balloons. This way aeroplanes could be worked in near the end of the tech scale in a way that really keeps them in a structure meant mostly to provide recon advantages to land or naval units. Near the end of the tech tree with the late ww1 era bombers like the Handly Page and Zeppelin bombers, I think that aeroplanes/zepps should add some tactical advantages as well, but I see the point of those that want to limit this to brigades.
However I think that strategic use of large fleets of bombers, though it should be very expensive, and hard to achieve too early on, is not all that a-historical. However the game could be made more historical by making large air units cost proportionally more industrial output than manpower, leaving a country that produced a large airforce with the only option for its manpower of producing untrained militia units. Since air units cant hold territory and ww1 bombing technology (which should be the cap in the game) should never produce more than a point or two of strategic or tactical damage, in the end I think this will be a failing strategy.
Air units should be much more influential in small numbers because of their recon advantage. Either half a dozen independent units of observation balloons, or one balloon brigade equipped division in each stack is all that is needed. More will be superflous and a waste of industrial output because of their high expense to combat effectiveness ratio.
As far as actual effects these will depend on the combat model, but should obviously start very modestly with early (1830s) balloons, or perhaps even not start at all till balloons became more widely used in the 1860s. I think that one big breakthrough is when the balloon operater could be connected to the ground station by a telegraph cable. At this point im not sure if actual game map viewing advantages are in order, it depends on the scale of provinces in Victoria and how far a baloonist with a good pair of binoculars could see. However perhaps there could be a tactical recon formula in Victoria sort of like the cavalry superiortiy formula in EU2, basically if one side has balloons and the other doesnt they get a bonus to the combat tables or die rolls or whatever.
Once planes are availible they can actually cross the lines and see a province or so into the enemy rear. Also planes can shoot down observation balloons. If air units are independant then this is easy to model, if not then probably you should just give more advanced airplanes a higher tactical recon value, and whichever side has the greater value (over a certain spread that would be considered a tie) will get the recon advantage. The downside of this is that early on in the game it will give a less than logical tactic of "balloon wars" since the side with more balloons would have an advantage while in reality basically you just need enough to cover your frontage and balloons cant really fight each other.
Still I think having observation balloons and airplanes in the game would be cool, but I dont want to see Handly Page bombers showing up in 1850 any more than the rest of you.