GTs definitely should stay as 3 weeks.
I could stand the Giro and the Vuelta swapping slots in the calendar every other year as long as the former doesn't lose the reputation as being "the hard mans GT" which it has developed. On the years they hold in late August/September they just need to find more unpaved roads etc..
... or they could probably move the Vuelta the back to start at the end of September, bring the WC forward a bit, can some of the early season races and accept that it is mostly different guys riding the early season spring classics to the GTS.
All proposals have many issues I can't think of a solution that would make everyone happy!