I mean not every planet obviously is gonna have an atmosphere like earth.
Type 1 - Earth-ike atmosphere with sufficient oxygen.
Gameplay-wise: No modifiers.
Type2 - Thin or low-oxygen atmosphere. While not immidieatly lethal, prolonged exposure to the outside air without an oxygen tank will cause harmfull effects. Gameplay-wise: infantry units should have their upkeep raised a little bit until some tech ("Advanced rebreathers" or something of that sort) is researched.
Type 3 - Poisonous, containing high amounts of particles harmfull to humans. Even short exposure is harmfull or lethal. Gameplay-wise the infantry upkeep part is the same as type 2. Also population growth is lowered a bit and population happiness is lower until a player researches a tech to counteract this (called "Biosphere city blocks" for example).
Type 4 - Toxic atmosphere. Not only lethal, but also dense and mildly corrosive.
Gameplay wise: All the effects of Type 3. Also all units (not just infantry) have their combat ability lowered (for example by -20% to both attack and defence. Aliens are not affected) to simulate equipment failures due to corrosion. This is in effect until a player researches a tech to counteract it ("corrosion resistant alloys" or somethign tot his extent).
Naturally the higher type atmosphere, the harder the game gets. But this is true for all factions, so it avoids balancing issues.
Suggestion 2 - Gravity types
Type 1 - Lower than Earth gravity. Workers are slightly more efficient, but population growth is lowered until the player researches an advanced (purple or gold level) technology ("artificial gravity zones" etc)
Type 2 - Earthlike gravity, no additional effects.
Type 3 - Higher than earth gravity. Workers are slightly less efficient and population growth is lowered until the player researches an advanced (purple or gold level) technology ("artificial gravity zones" etc).
Suggestion 3 - Vegetation levels
Type 1 - Barren. There are more arctic and desert hexagons, less forests.
Type 2 - Regular.
Type 3 - Lush. Less arctic hexes in colder areas, instead more thundra. Less deserts, more forests everywhere.
Suggestion 4 - Disasters
Volcano eruptions - A random mountain hex becomes an erupting volcano. for 3 turns it provides no resources and every city in range gets -1 to happiness for the duration. After that it goes back to being a regular mountain.
Typhoons - A typhoon hits a random area (9 hexagons perhaps?) centered on a coastal hex. All units insize the affected area lose some health (not enough to kill a full strenght unit, but enough to kill a wounded one). Lasts 1 turn.
Earthquakes - Similar to a typhoon, but is centered on a land hex and affects only land units.
Crop plague - lose 2 food for every city.
Meteor strike - a very rare event, hits 1 hex with a lot of damage though (like orbital bombardment)
Also the frequency of disasters can be set at the world generating screen.
Overall these suggestions are aimed at having more variation in world types, but (in case of different-than-earth settings) also giving the player the feel of beign in a hostile environment, which has to be gradually overcome by research. It also allowes veteran player to face a greater challenge - setting the world to "Toxic atmosphere, high gravity, barren, frequent diastsers" to try colonizing a really volatile inferno planet
