Pokemon is tricky because you have multiple generations with a lot of differing expectations, and a pent up demand for Pokemon in real life. Pokemon Go is a fun little jaunt and for it's first summer, everyone, casual or hardcore fan, was playing. But hardcore fans want something more which is why I've always put out ideas that make Pokemon more real and less virtual. Having a more elaborate version of Go is not what the majority of people would want, and that includes casual fans. I can already go to the park and use my smartphone to play Go and get the same effects we're describing without Universal investing in the experience. It wouldn't make me any more likely to buy merch because again, it's virtual.
I think you need a dark ride attraction and some AAs spread throughout the land. The Pokedex could certainly be marketed as an exclusive extension of Go, although Go kind of acts as it's own Pokedex already.
I have always been a huge proponent of an Ollivander's style Pokemon partner show with Pokemon being brought to life in the vein of Banshee puppets 2.0, and I think you need to incorporate Pokeballs somewhere in the land somehow that aren't simple plush figures. If they could somehow utilize the new Pokeball Plus joycons in an exclusive way (ie trigger effects or register and capture the limited Pokemon AAs) that would be fairly ideal but I'm not sure it's possible or practical.
I don't have all the answers and I understand that we're talking about a real world theme park here. But I can tell you right now, I'm a Potterhead, and if I hadn't been able to enter Diagon Alley through a wall, I might not have been as interested in going. And if my wand hadn't chosen me live, I probably wouldn't have bought it twice (before the interactive switch and after). I'm very casual when it comes to Pokemon games, and I would probably be happy with something small. But I have already heard friends talk about the possibilities for theme parks with Pokemon, and if it isn't done right, you'll lose a lot of potential revenue. That's my view anyway. It doesn't necessarily have to be complex but it has to invoke a sense of wonder for the Pokemon that inhabit it and it has to give us the feeling that we are seeing real deal Pokemon rather than dealing in the same virtual world we've had since the 90s.