Normally, having more than one oscillator in a small case is a but much, but for drones it's good to have an oscillator for FM usage or just running them together and getting contrasts from them. In a drone synth, at least two oscillators that can interact with one another will go a long way.
A filter might be a good addition as well. Make sure it's ergonomic for you.
There are a variety of good dedicated drone modules/synths, ranging from the Grendel Drone Commander 2 to the Eowaves Weather Drones to something standalone like the Stylophone GS-2. Even a synth not necessarily sold as a drone synth like a Moog Mavis (which can be both racked or kept in its own case) or other all-in-one device can be the basis of a great drone setup, and they have the benefit of combining a bunch of obvious tools in a smaller form factor so you can focus the rest of your case on more personal choices.
Overall, a "drone synth" is one of those things that really comes down to what you want. Can you think of examples of the sorts of sounds you have in mind? Drones can take many forms.