IOTL, Finland gave up its claims on Repola and Porajärvi (
see a map) after Soviet Russia offered Petsamo to Finland in exchange during the peace negotiation in Tartu in 1920. Petsamo being an Arctic Sea port, this offer was hard to decline for Finns and has been also the target of military expeditions during the Kinship Wars, a period in the late 1910's and early 1920's when Finnish volunteers travelled to Finno-Ugric areas in Kola, East Karelia, Ingria and Estonia to support/create local nationalist movements and aid efforts to eventually annex some of those areas to Finland. Compared to other attempted conquests in East Karelia, Repola and Porajärvi were in some ways special cases. Unlike elsewhere in East Karelia, people in those places actually did show interest in joining Finland and the Finnish government was already treating them as Finnish territories. Soviet Russia was also ready to negotiate about the future of those areas, unlike with other areas in East Karelia. Now, using some handwavium, let's assume that something happens during negotiations which allows Finland to gain those two territories, in addition to retaining Petsamo, and doesn't withdrew from the area in 1921.
For one, there would be possibly less talk about the ”shameful peace” of Tartu among nationalist right-wingers, though there would be probably many of those who would continue to think that Finland should have got the whole East Karelia. Finland being more succesful in Repola and Porajärvi might actually have an ironic effect to make some nationalists even angrier when the peace eventually comes, because unlike IOTL, ITTL these expeditions to East Karelia actually seem more viable and succesful. Bobi Sivèn, a right-wing activist, wouldn't kill himself after hearing about the result of negotiations and that his plans to arm locals in the area were denied and nationalists would lose one important martyr in their mythology.
Demography-wise, the area wouldn't increase the population of Finland that significantly. I couldn't find the excact population of the area, but it seems to have been around 4.000-5.000 at the time. Repola and Porajärvi had fairly similar population levels. Culturally the area is rather interesting, being Orthodox and speaking Karelian language, though probably counted as a Finnish dialect ITTL, so there could be small butterflies there.
I would assume that there would be some concerning the East Karelian Uprising 1921-1922 but I must admit that my knowledge of that conflict is rather limited. Repola and Porajärvi were two areas where Finnish volunteers were most active IOTL, but that wouldn't obviously happen here.