People should lower their expectations to a minimum with Fdev
this doesn't really go with that
If we gain access to richer planet types with water, lakes, rivers, oceans, clouds, rain, lava, magma, storms, erosion, forests, fauna I would pay a premium for that.

Creating all that is, in my opinion, an insane level of difficulty to develop. As E

is as scientific as possible simulation of a galaxy, there are a lot deeper and more complex logic to procedurally spread out assets than for example NMS. Even right now, after years of Odyssey - there are still severe bugs with planetary tech. Now imagine the level of complexity to simulate water, lakes, rivers, oceans, clouds, rain, lava, magma, storms, erosion, forests, fauna... you could also add a big game hunting many are talking about to that while we're at it, hehe
Something tells me that Frontier won't sacrifice that immersive & accurate simulation just to add pretty pictures.
Deeper gameplay and content would be a bonus.
On that I disagree. It is a game. Gameplay has to be there, otherwise it would be a huge investment to develop something that would only allow for players to take pretty pictures and not much else. All those memes about "mile wide inch deep" would come back with never seen before ferocity.
They cannot deliver lush worlds with loads of new gameplay in a short timeframe afaik.
Yep, that's why I've said initially - Not only I would buy it, but it would also be interesting to see dev videos, progress and discussions
over the years it would take to develop it!
Maybe they're secretly developing new planet DLC though.
Doubt that. But what they could be doing, with any future feature of the game - is to streamline and prepare parts of what's there for possible future additions and developments. If there are long term plans for Elite, of course developers could create prototypes of things, just to test ideas and pinpoint pain points in a more practical way. But any of that is nowhere near even ready-for-alpha state. Realistically, that's the best case scenario.
New planet types could offer more colonization options (buildings, resources, mining etc).
From what is knows right now, Colonisation could lead for Planetary improvements and expand to a more complex planetary environments. To me it looks pretty obvious, but only if to take into account the whole CMS experience of developers, engine and possible reusable code. And if Colonisation was successful, then maybe there wouldn't be a need for EA for 1-4, and these planet types would happen (maybe not to the fullest extent) as parts of Colonisation feature evolution, maybe even 8 (GGs) is a possibility.
The only problem I see with that - Heavy incline in CMS gameplay, and only a copy of existing open-world/space gameplay - not really my cup of tea. There will be Frontier Unlocked in a few weeks, maybe we would see if speculations like that have any merit.
5-7 is most definitely an expansion/DLC level content years and years of development away, or sacrifice that scientific simulation and do it quicker, but I don't see how it would make anything better as this simulation of our huge galaxy is one of the main features of Elite.