However, I do think that some of it could use some further enhancement, namely base fuel drops like in Lend Lease or Soviet Industry.
The Mechanic
Planes deliver the fuel, instead of automatic conversion.
Explanation: The counter-play added here is that the fuel is dropped by plane coming from the longest direction over the map, making it vulnerable to anti-aircraft weapons.
The Problem
Anti-aircraft weapons are hard to counter and don't require gameplay to use.
Explanation: The problem is that anti-aircraft mechanics are one of the game's weakest aspects. There is effectively nothing about it that requires player input. The counter-play is literally build an AA weapon and don't engage it in battle. OKW's abundance of AA weapons makes this especially easy and inevitable in team games.
The effort required by the Allied players to counter these AA weapons, and lack of ways to utilise the ability, means that using the ability is actually more about counter-counter-play, and one that feels heavily unrelated or detached. Bottom line: There's nothing fun involved.
The Solution(s)
Make it more like Supply Drop Zone, where it comes down to micro and not AA.
Explanation: Draw from the supply drop zone ability for inspiration. Whereas fuel drops can only happen in your base, supply drop zone can only happen on an applicable fuel/ammo point. These tend to be in contentious areas of the map by design. This means that there is a deliberate risk that the opponent may be able to use land units to steal that fuel (or ammo). These planes can't be shot down, so the counter-play comes down to the two sides fighting to pick up the fuel first (helped by the drop happening over an extended period of time). Making sure some sort of indication or notification of the drop is made noticeable by the opposing team, and you have well-crafted counter-play.
Because variety is the spice of life, you could potentially combine the two. Instead of limiting the fuel drop to the base, and making the plane direction always the same, allow the player to make the drop elsewhere on the map. This can make the plane direction variable to improve its chances, but at the cost of increasing the risk of the opposing team being able to steal the fuel once it is dropped.
Thoughts?