You can do way, way better.
Firstly it'd be a good idea to let us know how many days total you've got up your sleeve, how big your group is, what the mix is (all enthusiasts etc.) and where you're travelling. If you go to Japan just for theme parks you're doing yourself a disservice - it's one of the most gorgeous places in the world in terms of landscapes, people, culture etc. and if you were to go to Hirakata Park over spending a day in Kyoto you'd be doing Japan dead wrong.
Secondly, it's not all about coasters. Don't be a credit-whore, it'll become a disservice to your trip because stats don't matter, experiences DO, and Tokyo Disney Resort is one of the best experiences you'll ever have in your life. DisneySea in particular is the best theme park in the world and I'll happily die on that hill. If you're an enthusiast and you don't go to DisneySea, you're not an enthusiast, it's that simple.
You can do one day at each Disney park and be very satisfied. Again, take the "there needs to be credits" hat off and put the "I love great experiences, especially at theme parks hat on" and I promise you your trip will get a ton better as a whole.
If you can't take the credit hat off and you need to do something that's not USJ/TDS/TDL, the two heavy hitters are Nagashima Spa Land and Fuji-Q Highlands. The beauty about the former is that it's on the way between Tokyo and Osaka, so if you're committed or on a tight schedule you can make it work as you travel from port to port. With the latter, it's next to some sublime natural beauty (Mount Fuji, Fujiyoshida, Fuji five lakes etc.) and is also home to Eejanaika, which should be the #1 coaster to get whenever you visit Japan. Not doing Eejanaika and going to Hirakata Park would be a crime.
Again, this is all depends on how long you've got, but think of the above as your "first time must-do's". If you have more time, or come back again on a second trip, Parque Espana (beautiful place in the world, Pyrenees is there too), Tobu Zoo (Intamin Megalite with t-bars, 10/10), Studio Ghibil Park (stunning and gorgeous), Yomiuiriland (near Tokyo and filled with classics) and Yokohama Cosmoworld (good vibes) are probably all parks I'd consider visiting before Hirakata Park. You've also got Thunder Dolphin at Tokyo Dome and Gekion Live Coaster at Joypolis to checkout whilst in Tokyo too.