What do you mean by “drone sounds like nature”?
Eurorack gives the user a lot of ways not only to produce an initial sound, but to process and alter it as well. Besides a ton of different filters, there are wavefolders, resonators, LPGs, FX, etc., that you can use. There will be a certain amount of trial and error involved in putting together a system you’re happy with; expect to sell and replace some of the modules you buy as you develop a clearer idea of what you want. We all could list for you the modules that most aspiring ambient modular synthesists buy first, but I’d feel bad if you buy them, don’t like them, and lose money selling them off.
A few things I will mention, at the risk of being obvious, are:
A source of randomness. ScottMFR suggested looking into the Krell patch. Make Noise has a YouTube video on how to do a simple Krell patch on their 0-Coast semi-modular synth. A key component of that patch is the random voltage generator (similar to a sample & hold LFO). You could get something as simple as a module that produces random voltages; more interesting would be a sample & hold module plus a noise generator (and/or other sources) to feed it.
Slow modulation. Drones get really boring really fast if they are static; have a way to slowly change the timbre, volume level, etc., over time. Sloths is popular for slow modulation (can go down to 30-40 minutes per cycle), but there are other modules you can use that are perhaps more flexible, especially if you don’t need that degree of slowness.
VCV Rack. Try out modules on your computer before you commit to buying anything. Mutable Instruments modules are very popular (some might say too popular) for ambient, and I think all of their modules are available on VCV Rack.
A sequencer can be used in ambient music, but that depends on you. For example, maybe you want to do something quasi-tonal, and have your drone follow a specific progression; set up the patch to trigger the sequencer to advance a step once every couple of minutes or so. Or maybe have a short melodic fragment that plays every once in a while, with no immediate repeat/loop (or have it on infinite loop, but have it fade in and out, so it isn’t always audible). I like using randomness, but sometimes I feel like it’s a crutch for my lack of compositional talent, and I should try to impose a little more structure.
And something I’ve been considering, that may or may not interest you, is a harmonic oscillator. If you’re looking for something different from a standard VCO, a harmonic oscillator - one with separate controls and gate inputs for each harmonic - can be a good alternative. Slowly modulate the harmonics at different rates, maybe trigger individual harmonics randomly like separate voices - it all seems like a good idea in my head, at least. VCV Rack, here I come…