Considering that every now and then i see front/theater suggestion, here is one that IMHO is superior alternative.
Game progress:
Game needs technology tree like warthunder for every nation. Every weapon and vehicle need to have year of prototype (for possible gold order weapon) and year of service (for campaign unlock). Considering that some weapons were either available in small quantities after entering service, or are OP for game balancing reasons, we can further put star rating that is currently available in game for particular year/campaign. E.g. panzer 4 ausf. F2 is 6 star on Moscow (so beginning of 1942, btw bad example cause it was not present in moscow), but it is 5 star for Stalingrad (late 1942, early 1943) and 3-4 star for Berlin (panzer 4 ausf. J is 4 star for Berlin)
Equipping squad with unlocked weapons:
Now we can equip selected nation default squads with weapons (by default I mean like rifleman 1, 2, 3 or assaulters 1, 2, 3 ). Game would prompt available campaigns depending on the loadout you have selected. So you can have all kar98 and panzer 2 selected in loadout, but if you equip one soldier with stg 44 you can only play campaigns where stg 44 was available (e.g. Normandy and Berlin). Game would also calculate battle rating for every available campaign depending on your loadout. Idk if average loadout BR, or vehicle weighted BR or max BR for your loadout would be the best.
Campaigns:
Would be set of maps and regiments/uniforms for default squads. Every default squad would have real life regiment that was historically in that campaign with appropriate uniform.
Matchmaker:
Depending on the availability of campaigns and BR, MM would try to match you across multiple campaigns. You would be matched with +/- 1 (or 0.5, depending on the number of players available) of your loadout BR.
Think that this covers most of the players requests over the year. Unified grind, multi campaign MM, historical accuracy and balanced game for newbies and veterans. This suggestion could probably use more work, but basic idea is here.