Where to next? Help us decide

Tokyo Disney Resort is amazing, as is Japan as a whole, and is well worth visiting at some point. However, I would strongly advise against visiting it in summer of 2018.

Journey to the Center of the Earth will be closed for refurbishment from May 7 - July 20. It's maybe the best ride Disney ever created, and is not to be missed! Tower of Terror is closed for all of May.

Crowds on either side of those dates are Tokyo's worst. Unless you can go in September of 2018, maybe it's better to wait for another year.

I would suggest if you're doing Europe, Don't forget Alton Towers and Blackpool.

They are not day trips from London (London you're looking at Thorpe Park, Legoland Windsor, and Chessington world of Adventure... I have heard a convincing argument that the Tower of London was the worlds first Theme park).

Alton Towers is the premier park in the UK. IT has by far the best coasters, and a nice relaxed spacious layout.

Blackpool is a must if you want to see everything that Walt Disney hated about the Amusement park industry. Carnaval Barkers, rides that not everyone can ride, overcrowding on rides and limited theming. Don't get me wrong, I love the park for what it is, and it has perhaps the greatest collection of woodies and classic rides anywhere... but it is definitely from another time, and another... Ideology on what an amusement park is. Plus, hotel rooms are as cheap as anything in Blackpool

Be on the lookout for "Dungeons" operated by Taussauds... London definitely has one, I think Amsterdam does too (as well as Blackpool). These are great bite sized themed attractions (avoid the gift shop, its the mass produced rubbish you can get anywhere)..

While I've never been to Japan, I did a two week trip to Europe several years ago and it was excellent. On that trip, we spent the first couple days in London, then flew down to Venice and drove from there to Paris via Chamonix and Normandy. Unless you're going to London due to flight prices, I'd probably stick to mainland Europe if you aren't also planning to see more of the UK. While I liked the city, it is a bit removed from everywhere else on your trip, and you could use the time you'd spend sightseeing there elsewhere.

Because Europe is so large, my advice is to pick a few cities you want to see (2-3 per week of your trip), then look at what is near those cities or between them to decide which destinations to hit. If you're deciding your itinerary based on theme parks (which should be included, but probably shouldn't be the sole focus of the trip), I'd say Efteling, Europa Park, and Phantasialand should be your highest priority theme park destinations. Note that I haven't visited any of the above, but my friends that have done European theme park travels consistently rank them as the top parks in the region. I did get to visit Disneyland Paris on my trip, and I'd probably consider it as 4th in priority after the above three (it's not a bad park by any means, but you're probably better off focusing on more unique places than a French take on Disneyland). Note that getting to all these places will take time, so if you don't have much time for the trip (say less than 10 days), you may want to either do just Germany or just France and the Netherlands.

Go to Europe during the World Cup. Lots of local tourists will be on Russia or at home watching the games. Theme parks and other tourist attractions on Western Europe will be less crowded.

A couple of weeks in the UK, visiting the theme parks and other 'touristy' stuff, might be another option. As Chad mentioned, Alton Towers, Blackpool and Thorpe Park are well worth a visit. AT has SW8 opening in 2018 and that looks good so far. AT also has the best coaster in the UK (my opinion :) .. ) in The Smiler. Thirteen and Nemesis are also awesome rides. Blackpool is a typical English seaside town (with some great coasters) and is an experience beyond all description.
Thorpe Park has some good rides, and 1 great ride with The Swarm, but it does tend to get very busy, so I'd recommend an early start. I wouldn't recommend staying at the Shark Hotel (see the reviews) but the Alton Towers hotel was nice, and I'd stay there again.
Other smaller parks include, Chessington, Drayton Manor Park and Lightwater Valley (home of the Ultimate)
Along with a few days in London, you could keep yourself busy for 2 weeks in the UK as well as experience some world class coasters.

Thanks to everyone for all the input. We are currently taking all your comments in consideration.

@Douglas Hindley Any way to confirm this? This may be a deciding factor

Here's Tokyo Disney Resort's official refurbishment schedule, which still has Journey closed from May 7 - July 20, and Tower of Terror from May 7 - May 29.

And here is the best crowd calendar. It's only in Japanese, but assume any day with a number under "50" is OK crowd-wise.

What dates are you considering for travel? Also, whether or not you make it out to Japan in 2018, I would be honored to provide more detailed advice whenever you do.