Winner
Diverse in Unity
Yeah, if we already factor in demographic trends for the EU "big three", shouldn't France come out on top?
Population growth is notoriously difficult to predict accurately, especially now when developed countries went through the transition. Relatively small changes in the patters of migration, the overall fertility rates, life expectance, etc., can totally change the end result you get.
For example, the Czech Rep. was supposed to have less than 10 million people by now, or very close to that number. But in fact our population has grown to 10.5 million, largely due to increased immigration and slightly increased fertility rates in the past 8 years or so. I guess in the early 1990s nobody was able to imagine that people would actually want to immigrate here

Anyway, my point is that predictions based on current demographic trends should be taken with a grain of salt. Basing an economic prediction on an unreliable demographic prediction just increases the number of unknown variables, which makes the final prediction that more unreliable.
I think Germany will remain the largest economy in Europe pretty much forever. Once could argue it has always been the largest economy in Europe, at least since the end of the Dark Ages.
On global scale, it's impossible not to think China would've overtaken USA by then, by simply being four times their number. Same reason for India, basically, although it's not as certain that they'll be that far in 2050 already.
India faces huge challenges. I think it will come close to a full internal collapse in the next 40 years.