OK i think your onto something so let me add a few issues here.
Artilery is a range unit so defenders must detrench to engadger them this pulls infentry out of there positions ingame this means artilery without infentry acompinyment are basicly sitting ducks after the first strike. No defence
Artilery is mobile moveing like Faster tanks without the defence or ability to capture a city
Artilery is Highly targetable from air units But also can attack air units. here they need to be week and strong at the same time. For example 1 bomber kills 3 artilery But you also loose your bomber 2 bombers you keep a bomber.
Artilery is hard for infentry to find without air suport. Bonus vs infentry on defence side when not acompinied by air units
Artilery as a mobile unit are basicly like transports untill set up and implaced so if you move an artilery its attack drops low but on your next turn it would now be implaced so it would go back to normal att/def
Artilery is most like infentry in defence and Tanks on offence with range
Here is a proposed Artilery Unit
Range 2
Move(1 faster then tank)
Att Mounted 3 Def mounted 3
Att Implaced 6 Def implaced 4
+2 def.when acompanied land unit while emplaced
-1 defence vs plains
Stelth to all ground and navel in range like sub looses stelty if moved.
Free round of attack for each range space away from defender
Spoted by planes from 7 spaces and from Spy plains at 12,