It takes very little to be better than tha Harrier...
The Harrier was a nice plane, its VSTOL capabilities a first, but it could never be a great fighter or attack plane in a difficult environment exactly because its VSTOL capabilities made it a slower, more vulnerable plane. Good for CAS, if you already had air superiority - but it couldn't obtain it alone against a capable opponent.
As a VSTOL plane, and Harrier replacement, the F-35 can be somewhat better, if they can tame and fix its many teething issues.
Just, as an all-around plane - one that should take the roles that were covered by F-14, F-16, A-6, the F-35 is just a so-so plane, and will have troubles against more capable ones.
In fact, USAF didn't want to renounce to its F-15 and F-22. Just, everybody else has to get the F-35.