I have some suggestions for some modifications of Spitfires, that might be interesting as either Premium or event vehicles.
The 3 Spitfires with the serial number BP985, BR114 & BR234, were all three Mk Vc, that were similarly modified by 103 MU, to be able to intercept the high altitude Ju 86P. From what i know, they engines were modified with a higher compression ratio, the propeller were changed to a 4 bladed de Havilland propeller and some of the armaments were removed to save weight (I have found conflicting information on what armaments they retained, some said they kept the cannons, some said they kept the machine guns and one site i found even said they were equipped with .50 cal). They successfully intercepted a Ju 86P at an altitude of at least 39.000ft. One of the pilots of these 3 aircraft is George E. C. Genders, one of the pilots that died during testing of the DH108 Swallow.
Image of BR114
History of BP985, BR114 & BR234
BP985:
FF 10-4-42 6MU 12-4-42 47MU 20-4-42 SS485 9-5-42 India Middle East 1-8-42 mods at 103MU Aboukir for high alt interception Engine cut forcelanded on beach and overturned El Gamil 7-10-44 SOC 27-10-44
BR114:
FF 12-3-42 39MU 14-3-42 Crosby Co 23-3-42 mods Alderamin 4-5-42 Takoradi Middle East 26-6-42 CB ops 13-9-42 mods 103MU Aboukir for high alt interception MedAAF 31-3-44 Armee de l’Air GR2/33 11-7-44 destroyed by Hak 31-8-44
BR234:
FF 27-5-42 39MU 28-5-42 82MU 10-6-42 Nigerstown 20-6-42 Takoradi 28-7-42 Middle East 1-8-42 SOC 13-9-42 M61 high alt fitt plus other mods at 103MU Aboukir for high alt interception
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The second spitfire modification is a Mk IX with the serial number BS273 that was also modified to intercept the high flying Ju 86R aircraft. The modifications on this, included removing armor on it, and reducing its armaments to only 2x 20mm. It was piloted by Emanuel Vladimirovich Galitzine, where with this aircraft, he is credited with the highest recored air combat during WW2 at 43.000ft.
Image of BS273 (It says BF273, but it was repainted wrong by ground crew)

History of BS273
FF 14-8-42 R-RH Cv IX M61 64S 16-8-42 AFDU Duxford 16-8-42 Cranfield 3501SU 27-8-42 Special Flt Northolt 5-9-42 Wood prop armour removed armament reduced to 2x20mm can lightweight pt finish operated under code Windgap reached height of 45000ft 124S 25-1-43 3501SU 27-3-43 453S 1-5-43 mods AST 9-5-43 453S 15-5-43 ? AST riw 24-6-43 453S 26-6-43 Shot down by fighters nr Cayeux 4-7-43 F/Sgt C Woodall killed
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Then there is the AD269, where it says that it was a trial Spitfire Mk Vb with 2x20mm cannons and 4x5in mgs.
History of AD269
Trial Spit with 2x20mm can 4x5in mgs 24MU 6-10-41 317S 13-10-41 AST 8-12-41 CAC ops 14-4-42 315S 4-9-42 303S 30-11-42 Crashed in bad weather Normandy-le-Wold Lincs FACE 15-12-42
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There is also NH346, which is a Sptifire LF Mk IX with D type wing carrying 2x 20mm and 4x.5in m/gs.
History of NH346
with D wing (2x20mm can and 4x.5in m/gs) original FV fin and rud 45MU 22-5-44 412S 22-6-44 1ATC 24-8-44 AST 1-9-44 485S CE ops 15-11-44 bboc 28-2-45 MedAAF 3-3-45 87S ‘LK-M’ SOC 30-4-47
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Of these aircrafts suggested, i feel as if the BS273 & one of the 3 Spitfires modifed by 103 MU would be quite viable to add into a tech tree, because the information available about the aircraft. But for something like the AD269 & NH346, while they would be interesting aircrafts because of their armaments, i just haven’t really found any of info on them outside of the history of the aircraft in regards to unit and squadron history and its fate.