For me, they need a good quality dark ride and a proper water ride (log flume or river rapids) and the current temporary log flume needs to be scrapped. It's the only real eye-sore in the whole park and it's really got a "funfair" feel when their latest investments are definitely trying to fit into the theme park market rather than an amusement park.
I'd definitely say water ride and a dark ride before the next coaster. They've just built 3 coasters in succession over the last few years, and whilst you can never have too many coasters, I feel like they should focus their attention to add some variance to the ride line up in the park. An indoor complex area with a dark ride and a couple of smaller flats/play area and restaurant (similar to the one at plopsa, or at legendia) would be a good move in my opinion, as the park really needs an area with attractions for when the weather isn't great, as the whole park is outside.
Aside from that, in the future retheming cobra and edge into a brand new themed area would be really lovely and they need to replace the Main Street with some properly detailed facades, at the moment it looks a bit cheap. Magma can stay where it is, perhaps could be integrated more into lost kingdom, but they are definitely on track to creating a proper theme park.