They can make new styles fine, it's a matter of how they implement them and they have to be very careful.
When it comes to tanks, SE is trying very hard not to have an FFXI ninja problem again, where you only tanked as ninja or you didn't tank at all because it was so glaringly better than any other tank out there. It was an evasion style tank. lifesteal tanks are equally as difficult to balance correctly. Blood death knight tanking in WoW has either been really underpowered or really overpowered, no in between.
People want new styles of gameplay but they don't want years of broken mess for the sake of it.