Timeline is ok as it is. Actually I don't play from 1399. As it has said above, I think it would be a great idea the rise of Austria as main thematic.
And about game mechanics, I'd like something done about economics, making them deeper. It's time of the beginning of companies of trade (colonies and proto-markets in America and Asia...) and it could be nice to see the development of "global traders" (Spain, United Kingdom, Netherlands...) vs "huge local producers" (France, Russia...); and the different results of "public" economies (depending more in nobility, like Spanish viceroys and latifundists, or in State, like enlighted France) vs "market" economies (England or Netherlands...).
Economical basis should have different consequences on societal and political issues (remember, why industrialization began in England and Netherlands...). Taking into account even Max Weber ideas about the influence of Reform (religious and cultural issues in fact) in the "spirit" of capitalism.
It could be a good idea... or it's just too hot in Spain
