I understand the rationale for a 1523 spawn—it represents the birth of modern Sweden, the breakaway from the Kalmar Union (which would be represented by a united Viking Scandinavia in game) and the end of Danish domination. Yet I feel an earlier spawn date would be better, since the Swedish state did exist for centuries before 1523. It actually started to colonize Finland and Norrland in the Middle Ages. In addition, the Kalmar Union wasn't very stable, Denmark and Sweden kept separate identities, and they were even in open conflict during the union.
Unfortunately it's not clear when the kingdom of Sweden actually started, but a date that could work is the foundation of Stockholm c. 1250. This also corresponds approximately to the end of the consolidation of Sweden into a unified kingdom (see
this guy).