I think everyone in this community will like these ideas (or at least it is my hope you guys do) however I know everyone has their own opinions on this matter so please comment and share your own ideas about how to approach this issue of playable characters in the game. As examples I have posted some pictures below, but I understand these are just examples and cannot/will not be copied into the game exactly as they are for obvious reasons, although I think they are useful to spark discussion and maybe inspire future designs.
Brief:
Playable characters in Simplerockets2 can be simple Mechanoid (android or robotic) “blocks”. That is, a torso and a “head” with very simple features. Limbs can be added to the torso in Designer Mode as parts or even constructed by the players themselves, giving a great amount of freedom as to the abilities and design of each android.

-Each Mechanoid can accumulate experience during career mode, and function as a “Command Chip”, copying its own “Activation Groups” onto a craft’s existing Command Chip.
-Mechanoids do not need air, food or water to survive, though they will need to recharge their battery (in their torso or installed separately) at a charging station connected to solar panels, larger batteries, or reactors.
Possible names:
If the characters will be robotic or mechanoid in nature, names could include “Androos”, “Androods”, “Droos”, “Drooyans”, or any other that the community likes or can come up with.
Facial Features:
There are many actual robots on the market that use only eyes to convey “emotions” and as it is simple and yet can give the android “emotional character” I think this is the best way to approach the issue of facial expressions.
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Locomotion:
Easily achieved by the devs adding a chain track part or otherwise players can build android chassis out of existing wheel/strut parts, giving complete freedom of choice to the player for its design. Also this eliminates the devs having to animate walking motions.
Other limbs:
A grasping tool or “arm” would be very useful if the Devs can make one, otherwise players will have to devise one, although this might get complicated for players (and add to a craft’s part count), so I think a removable arm part should be introduced along with the Mechanoid torso “character”.
In addition, if the Devs make other parts that function as scientific instruments or repair tools [for example instruments or tools used in career mode] these tools can be placed on both player-made rovers or the Mechanoids. A player can then have different Mechanoids during a mission, all with different instruments or tools on their limbs, making them useful for specific tasks, and perfectly meshing these “playable characters” with career mode.

You can just make one using mfd