I don't believe there is a strict natural progression of RC plane types. However, in a general sense, there are natural learning steps.

For most, some forgiving type of plane is best for learning. How forgiving, depends upon the pilot. For some this may be a high wing trainer. Others may do just fine learning the basics on a low wing plane like a 4star. A second hotter trainer really isn't needed in most cases if the pilot mastered their first trainer in the first place. A person's second plane then should give the pilot a taste of what aspect of the hobby they wish to explore. Aerobatics: get a small aerobatic plane. 3D: Get a fun fly type. Scale: Get a semi-scale plane. Etc.

This point was mentioned in some previous posts. Many solo, and after a week they decide they need their second plane. I understand the enthusiasm, but rarely has a pilot really mastered their trainer before they look for a second plane. I think it is just human nature to want to move to the next level before one is ready. IMO, you really aren't ready for your second plane until you master cross wind landings, slips, rolls, spins, etc. and the pilot shows the ability to manage throttle and can use rudder. For most pilots this takes a whole season or more.

I think this is one reason why we see many levels of trainers. It really takes a long time for most to master the basic flight skills. People want to reward themselves for reaching milestones, like first solo, etc... but since they really haven't mastered their trainer yet, they simply get a little bit hotter trainer. There is nothing wrong with this, but the second hotter trainer really isn't needed IMO.

Cheers!