Ok, so I've got all VIP seats, Exec seats, all upgrades for Fitness center, as well as Training hall...
What should I build next? At the end of the season I will have a significant amount of money+prize money (around 3 million).
Is Youth Academy worth buying? Or should I simply build more and more normal seats at my stadium? If you think that Youth Academy is worth buying, please tell me the advantages of having such a facility.
Or maybe I should just keep saving money for a kick ass quarterback? I am quite satisfied with my players... I've got +4* talented players with very high consistency and intelligence on all important positions (QB, MLB, SF), but still no 5* talented player.

I agree, get the club shop and expand the stadium. The money can be used for two different purposes: 1.) Spend on better players 2.) Save up to pay for your players in future seasons as you train them.

The youth academy is pretty worthless in my opinion. I sold mine a few weeks ago and I'm still getting just as many youth pulls as before and they are just as good/bad as before as well.