Yep this is exactly what I did.
I made a long list of both the various IPs that I wanted to include, and the various themed environments that I thought were missing from the resort and should be represented. This was a huge list of both.
Then I stared at that list for a long time to brainstorm relationships to somehow find a park theme and organization. And then that concept for a theme further refined the original list of IPs and locations into a new more condensed list that could become a single logical park.
There's a balance point in the design process between the IPs defining the park and the park defining the IPs. Think at it coming from both at once to hit the sweet spot. It takes time, but having a plan is worth it.
Also, now that I see you are all starting up brainstorming for a next gate, I want to drop in a couple personal strategies I think about when I come up with a park concept. These ideas were less applicable to a Disneyland style park, but now I think they could help. Both are from longer blog posts I wrote last year that I'll link.
1. Parks have a theme and parks have an organization and they are two completely different things. The theme of a park is usually the simple, one phrase description, often a category, the organization is how the category is presented. You can use the difference to your advantage.
http://imagineerland.blogspot.com/2016/07/the-meaning-of-disneyland.html
2. Use IPs to benefit and strengthen a land, not to contradict it. My 3 rules for if an IP works in a land: is it popular enough to be worth the massive investment to build, does it match the physical setting of the land, and does it match the character and meaning of the land. If you don't get all three, its going to stick out, and inevitebly break down the theme that you try so hard to create.
http://imagineerland.blogspot.com/2016/07/my-rules-for-ips-in-theme-parks-or-why.html
And last point. A bunch of comments have suggested a SEA meets Animal Kingdom meets Disney Sea Park. Uh. Here you go.
http://imagineerland.blogspot.com/2017/02/disneylands-third-gate-disneys-worlds.html