Without a big in-house team, I’m not sure how I would be able to create one of these “monsters” 
I am a one-woman-show and it would take me at least a year to develop something like that.
Also, it does not seem so lucrative anymore. There are lots of the established mega-bundle 100-demos multipurpose themes out there already and they sell very well. I mean… 70 sales in a week? For a year or two? WOW!
But vast majority of the new ones, although well-designed, don’t have such strong sales and can be compared to a well-designed niche minimal theme in terms of sales. I feel as though this multipurpose category might be oversaturated.
I’d start with something smaller, very well-designed, well-written and unbloated. Good luck!