To make career mode accessible to more players (who's less interested in building / programming), there could be a piloting contract trees allowing players to complete the mission by piloting stock craft, unlocking launch site, tech nodes and additional stock crafts in the process.
Player could select from a few stock crafts or build a new one to complete the contract.
This could also motivate players to upload vanilla craft that balances aesthetic, handling and part count( even include Vizzy to improve handling or functionality). Even a few official events could be organized to gather stock craft design from the community, further expand the stock craft library in game.