Well, the first thing is just the nature of the Lightning.
It may finally be a competent aircraft, but it's still more of a gunship. We can definitely improve it with revisions, but it'll still be a gunship and it'd most likely be impractical to try to use revisions to change that.
If we ever want a dedicated fighter, we'd likely be better off designing a new fighter rather than trying to revise the Lightning into being that better fighter. And the expense credit won't apply here. That and we still don't know the effectiveness of the Lightning yet. While it'll definitely be effective, I'd rather wait and see if it's effective enough to warrant a credit.
That and the National Effort design -> Very Expensive is a very attractive idea.
Also, new potential design.
Potential Design: AS-HA2
The next evolution in artillery warfare, the HA2 introduces many new benefits that will cement its title as the successor to the HA1.
The HA2 is constructed utilizing crystal, and possesses a crystal-glass "shield" extending a bit outward and curving slightly inwards to protect the crew from (upper-)forward projectiles. Mechanisms allow for angling the barrel up/down easily and more-so than the HA1. Behind the shield and under the barrel is a much smaller Aether Reactor Mk. 2. This reactor is obviously similar to the one located in the Lightning, but is much smaller to avoid unnecessary expense, size, and weight. The Reactor does generate more power than needed, but not by a large amount - enough for future expansions (such as fabricating crystal inside the barrel).
The biggest change in the HA2 is the propellant.
Blastballs have long been outdated by the KPD. The KPD4 utilizes directional constant thrust that can be easily changed and its thrust distribution can be changed in real time by a human operator. Compare this to the Blastball - a single set-power unfocused (and weak) explosion.
The HA2 is moving to the future by incorporating the KPD4 instead of Blastballs. Perhaps the biggest advantage of the KPD4 is its more-directional thrust. This allows for us to greatly increase the power of the propellant without having to worry about the structural integrity of the barrel. Though of course we won't be using constant thrust, and will be needing a significantly smaller power input than that required by the Lightning.
The KPD4 propellant will allow for a much faster and longer-ranged projectile, and the scaled Aether Reactor will allow for no reliance on wizards.
A small set of dials located behind the shield allow for the crew to configure the KPD4 to allow for much more dynamic, quicker, and easier aiming. The power can be changed to accommodate for different ranges (and potentially ammunition types) without having to adjust the barrel at all.
Reactor - A downscaled Aether Reactor Mk. 2 allows for the HA2 to operate without wizards present.
Propellant - Instead of using Blastballs, we adapt the KPD4 to work as a propellant. Its directional thrust and far-greater strength (the KPD1 was an equivalent of 4 blastballs/second) allows us to invoke far greater velocities on our projectiles and not worry about the integrity of the barrel too.
Crystal - The HA2 is made of Crystal, and should get the Crystalworks bonus.
Shield - A simple crystal-glass shield extends slightly outwards from the HA2 and curves inward a bit to give the crew cover from projectiles.
TL;DR - We adapt the KPD4 and Aether Reactor to the HA1 to create the next generation artillery piece.