I think it makes more sense that America and UK start with carriers in the blitz scenario. Clearly, the requirements for building a carrier should be after level 2 cruisers and not battleships. However, they did have armor and the troops did have minor weapons emplacements for defense. A surfaced sub (for torpedoed attacks) was indeed vulnerable to surface defensive fire thus carriers should have a small defense against subs.
A viable option (playability wise) ought to be to convert units (they would have to be sitting in a factory province). A cruiser could be in 'dry dock' at a port and be able to convert into a carrier (and back again if need be) where it would exchange it's firepower for airport capabilities. Though a medium tank couldn't change into a heavy tank, it could (at a level 2 factory) be converted to a mobile artillery (and back again) of the same level. A light tank could be converted into an armored car (i.e., the armaments are sacrificed to gain more speed).
These conversions would each need to take time, have available space in a production facility, and cost some resources (or even gain resources like with the light-tank-to-armored-car conversion). I think that the manpower maintennance costs would need to be unchanged (or only slightly changed) so I think this would be a fairly reasonable limiting factor in determining which unit types can be converted.
Also, instead of just willy nilly conversions happening across the board, perhaps a research addition should be required. For example, to convert any level light tank to an armored car would require the "Armored Car/Light Tank Conversion Schematics" tech researched (perhaps a smaller tech around day 8 and has pre-reqs of level 1 of armored car and level 1 light tank. Also, to save space on the chart, this tech might share a row with the Armored Car section of the armored classes.
I have been having fits on occasion at the lack of carriers. But one would need to have special limitations. For example, all carriers level 1 - 3 would have to have less capacity than levels 4 - 5 (or whatever counts are decided). And you ought to be able to have a combination of fighters and one choice of bomber types. The limit on fighters should be up to five but sacrificing two fighters to make room for one bomber (so you can have at most 5 fighters or else 1 fighter and two bombers).
Also, non-naval bombers can't land on an aircraft carrier, so to simulate this, an aircraft carrier can only load non-naval bombers at port. Once launched, non-naval bombers taking off from an aircraft carrier can not return to the carrier. Thus, if another airport is not within range of it's target (note I didn't say within range of it's launch point) then the bomber is said to be on a suicide mission and must take on the nuclear bomber behavior.
On that last note, whereas tactical, naval, and regular bombers take up two fighter slots, a nuclear bomber would take up four. This way, a carrier will always have the ability to host at least one fighter and can't carry more than one nuke bomber (which would be serious overkill, otherwise).
Jet fighters would be a special case also. They would take up only one fighter slot, but would have to follow the rules of a non-naval bomber for loading and re-basing. And since they have such short ranges to begin with they would be a less-tempting suicidal unit. I don't think it would be reasonable to make it possible for a level 5 carrier to be able to start landing jet fighters as the era is still wrong. Realistically, jet fighters had a great amount of difficulty landing on converted Hornet class carriers after the war (thus necessitating the Kittyhawk class carriers later on). (And arresting wire technology couldn't handle the faster landing speeds either.)
Finally, on this whole 'scrambling' issue that has everyone's bee in a bonnet. There is a very simple solution to the reality of scrambling air units and it is this. An air unit's ability to scramble was based mostly on the surprise of the attack. To simulate the possible surprise factor, each unit should have a chance to scramble and a chance to fail getting off the ground. For example, a fighter might be given a 50/50 chance to take off. Once it has taken off, however, it must be considered to be re-basing at another airport (if one is within range) or else it must be destroyed. Returning to convoy mode AFTER fighting in a pitched battle for up to 15(?) minutes is completely unrealistic as the airfield is not usable for landing DURING a fight. If the battle is over in less than 15 minutes, the fighter can land. If the battle is not yet over, the fighter must re-base elsewhere or crash.
This is so obvious but has anyone considered it before? Also, tactical bombers and nuclear bombers (if they manage to scramble) should not be allowed to fight this battle and must immediately re-base (or crash if there is no base within it's range).
Finally, the odds of scrambling should depend in part on the class of aircraft (and slightly better odds for level (at least for the fighter) ). A fighter should start at 50/50 but a level 5 fighter might have a 10% improvement which would make it 55/45 chance. A regular bomber might be 40/60. A naval bomber might be 45/55. A tactical bomber might be 30/70. And a nuclear bomber might only be 15/85. The different odds would simulate size of craft, time needed to fuel up enough to escape, and cargo loading times (one doesn't send up a bomber without bombs, though the realism of that part may be debatable).
I almost forgot, if you have jet fighters, perhaps they could have a 60/40 odds of successfully scrambling.
This is my '32' cents since I've taken Ball's 30 cents and added my own 2 cents.
;-]

It seemed like such a waste to destroy an entire battle station just to eliminate one man. But Charlie knew that it was the only way to ensure the absolute and total destruction of Quasi-duck, once and for all.
The saying, "beating them into submission until payday", is just golden...pun intended.
R.I.P. Snickers <3