The problem there is that this becomes a self-fulfilling prophecy -- If you take the park that most people would call quintessential and then expand and add to it, it's only going to become more popular and outshine the other parks further. The Magic Kingdom already has the most high capacity E-Tickets of any park in Florida.
If Epcot had a menu of 25+ attractions all up and down the A to E Ticket scale, 7-10 of which were considered classics, and all of them adhered to the themes of the park, I don't think it would have as a hard time pulling people in. It might still be seen as secondary to the "Castle Park" experience, but it would be a much closer second instead of a diiiistant second.
Same with Hollywood Studios and Animal Kingdom -- wouldn't you spend more time at each of them if they all had as much to do as MK? Wouldn't they feel as essential to the WDW experience, or at least much moreso? Animal Kingdom actually leapfrogged Epcot in visitation last year, which is due in no small part to that park being expanded with great new attractions and spaces. I'm sure we can count on Hollywood Studios to take up that mantle in 2020. But the park is so wildly under-built at the moment that it will be massacred under the Star Wars crowds. Hopefully it will push them to get serious about rounding out that park with a bigger attraction menu.
I love the Magic Kingdom and would love to see it grow and add new attractions, but it's already sooo far ahead of the other three parks that I think they need to really get serious about building them each out into full experiences before they can shower MK with more gems. It has plenty to do, but too many people who want to do those things. Give those people that many things to do at the other parks and the growth pattern should start to level out. Otherwise it will be a continual issue of building more to up capacity at MK, only for more people to come for the new stuff, which then needs more capacity built to handle those people . . . while the other parks continue to struggle in comparison.