Theres are 2 simple fixes:

1) Halve exp and double or triple gold earned. This will solve some gold issues while slowing down fate grinds to be on par with other types of leveling

2) Make exp start to slowly lower the more people that are present. This would cause people to use multiple zones instead of just 'the most efficient'
ex:
1-24 people = full exp
24-50 people = 75% exp
50-100 people = 50% exp

I feel it would be enough to slow down zerging