^^ I don't think they can incorporate characters from 1 and 2, (Also the characters from 1 aren't all that good to begin with) because from what I've heard 1/2 are in their own timeline or set far in the past of 3/4. Honestly, I don't want newer characters because they might ruin the narrative. Labrys was surprisingly cool with her accent, but I don't think you need to jam new characters just because when there's still some left and I honestly would like to see what ever happened to the rest of the P3 cast in their later years.
Well I know for sure that 1 and 2 do in fact take place in the same timeline as 3 and 4 as there is ample evidence in 3 that they're interrelated, in terms of time difference I think it was about a decade or so between 1 and 4 (though i'm not particularly clear on the exact amount of time). Granted though you are correct that 1's characters aren't all that strong especially when compared to 3.
Additionally depending on how you interpret the true ending it could make logical sense to bring back just about anybody really depending on how Atlus goes about continuing the story and whether or not the interpretation I have and others have made of it is correct.