The expansions will include significant new features such as seamless freeform (manual pitch/yaw/roll) atmospheric flight and landings and on-foot and out-of-ship activities (such as FPS combat, walking around and boarding ships, walking in stations and walking and driving vehicles on entire 1:1 scale populated living planets), multi-player crew and player executive controlled capital ships.
Frontier intends to release small, free updates after launch. Expansions that include significant new features and content will have to be purchased.
For example, the roadmap is to add these features over the following expansions (in no particular order): 
- Landing / driving / prospecting on airless rocky planets, moons & asteroids (Elite Dangerous: Horizons)
- Walking around interiors and combative boarding of other ships
- Combat and other interactions with other players and AIs in the internal areas of star ports
- Accessing richly detailed planetary surfaces
- Availability of giant ‘executive control’ ships to players
- The scope for Elite is huge, so a sensible strategy is used to add to the game in stages.
Keep in mind that the game has been well planned and designed from the start with all these features in mind (it is not an afterthought) and Frontier Developments already has the technology for this.
- The Future of Elite: Dangerous
- Elite: Dangerous - Development Plan video
- Q&A about Development Plan
- Newsletter about Landing on Planets, Ship interiors and Walking around
- In-depth Yahoo interview with David Braben about the scope of Elite.
- In-depth Edge article with David Braben and Jonny Watts about the scope of Elite.