This suggestion draws heavy inspiration from PilotAwesome_FIN’s suggestion found here:
I highly recommend reading that before this, as I base my suggestion on his.
First my disagreement with him: I don’t think it is a very good idea to implement all of the Finnish wars into one, and that is why I only suggest this one campaing about the Soviet Vyborg-Petrozavodsk offensive in the summer of 1944.
History:
After the Finnish offensive stage had ended in 1942, the Soviet Union and Finland had been fighting in WW1 style Trench warfare. In 1944 The Soviet leadership decided to push the pesky Finns back to where they came from before dealing with the Germans. This was the Vyborg-Petrozavodsk offensive. In 10th of June the Red army attacked the Finnish positions in the Leningrad front, starting the tough fight for survival. Vyborg–Petrozavodsk offensive - Wikipedia (If you don’t read any of the other links, at least look trough this one)
These are some of the Battles/Maps that could be represented in the Campaign. They all had pretty much the same basic units fighting in them, but with some difference:
On the 13th happened the Battle of Kuuterselkä, which caused major losses the the Finnish Assault Gun Battalion (Rynnäkkötykkipataljoona) destroying 5 StuG III G:s (Total of 8 were lost during the Soviet Offensive). The Finnish tanks (according to the Finns) destroyed 11 T-34s, 3 KV-1s and 3 Assault guns. Kuuterselän taistelu – Wikipedia
On the 21st of June the Red army attacked Finnish positions at the Svir river. The attack was stopped at the Finnish U-line (U-asema/U-linja), with the most significant battle was the Battle of Nietjärvi in 10-17 of July. U-line was heavily defended, consisting of multiple firing positions and 12 concrete bunkers. Suurhyökkäys Syväriltä 1944 – Wikipedia, Battle of Nietjärvi - Wikipedia
Battle of Vyborg happened in the 20th of June, with the city being lost to the Russians in just 5 hours. In Vyborg happened the last appearance of the infamous BT-42. Finns lost 23 soldiers with 420 missing and 8 BT-42s in the battle, and the rest retreated to Tienhaara where the Soviet bush was stopped a few days later. Viipurin taistelu (1944) – Wikipedia, Battle of Tienhaara - Wikipedia
25th of June started the Biggest battle fought in the Nordic countries, the Battle of Tali-Ihantala which lasted until the 9th of July. The Soviet forces attacked the finns with 150 000 Soldiers, hudreds of tanks and aircraft. The Finns had 50 000 soldiers at the start of the battle, and about 100 000 by the end of it. Finns lost 3 tanks and 12 aircraft, Germans lost 33 aircraft and the soviets lost (according to Finnish estimates, so not very accurate) 600 Tanks and 284-320 Aircraft. Battle of Tali–Ihantala - Wikipedia
Another major defensive battle happened on the 21st of June-17th of July, the Battle of Äyräpää-Vuosalmi. This battle had some of the toughest fighting of the war, with 20 000 Finns desperately defending against some 60 000 Russians. Battle of Vuosalmi - Wikipedia
Special units:
Different volunteer units, like Swedish or Estonian volunteer squads
Detachment Kuhlmey. German luftwaffe unit operating out of Finland. It was equipped with Ju 87D-5 Stukas, Fw 190 A-6, F-3 and F-8 Fighters and Me 109G-8 recon planes (Detachment Kuhlmey - Wikipedia)
Sturmgeschütz-Brigade 303. German Assault gun brigade which fought in for example the Battle of Tali-Ihantala