I know so many people are saying "it's too soon to talk about 4th park!" I think 10 years is a solid gap between new parks and LOTR fits perfectly within that timeline.
1) Universal started planning for IOA relatively soon after USF, and opened within a decade (9 years).
2) Universal had a 4 years gap between opening Hogsmeade in Orlando and Japan. And a 4 years gap between SNW Japan and Orlando.  If they're bringing LOTR to Universal Great Britain in ~2031, realistically we could see LOTR coming to Orlando 4 years after that, which lands us at 2035.
From what I can tell, there's no good reason to wait to build a Wicked land, other than to finalize things on the Creative side.
Epic's biggest issue right now is the lack of things to do, long lines, and overall park capacity, all issues at least partially solved by building expansions. A new ride or two is fine, it would definitely help, but even moreso what would help is a new world. And building a new world does not prevent them from building a new attraction or two at the existing portals. Look at USF from 2012 to 2014 for precedence:
Jan 2012: Jaws closes to make way for Diagon
July 2012: Despicable Me Minion Mayhem opens
June 2013: Transformers opens
August 2013: Springfield expansion (including Kang & Kodos) opens.
July 2014: Diagon opens
And yes, USF needs help, but realistically we may not see Simpsons close until 2028, if it closes at all. So that new world may not come until 2032 at the soonest. Universal could act much quicker on other plots of land at USF. For example, Supercharged is up for replacement, Fear Factor has been on the chopping block, Blue Man / Nickelodeon could be better utilized, there's potential expansion near KidZone, Fallon could be replaced, etc, there's a ton of potential at USF without looking at Simpsons.