Sketch of how to create a WPILIB command that can be used in a group but which isn't created until initialization

OnTheFlyCommand.java

// This class allows you to use a command that freezes configuration on construction that you would like to be deferred
// until initialization.  For example, you might want to do path following with a trajectory you generate based on the 
// current position.
//
// Example usage:
//
// MyAutoCommand extends SequentialCommandGroup {
//     public MyAutoCommand(...args...) {
//         Callable<Command> c = () -> { return new MyPathFollowerCommand(...args...); };
//         addCommands(
//             OnTheFlyCommand(c),
//             ...more commands...
//         );
//     }
// };

class OnTheFlyCommand extends CommandBase {
    Callable<Command> m_commandCallable;
    Command m_command;
    
    public OnTheFlyCommand(Callable<Command> commandCallable, Subsystem... requirements) {
        m_commandCallable = commandCallable;
        addRequirements(requirements);
    }

    @Override
    public void initialize() {
        m_command = m_commandCallable.call();
        m_command.initialize();
        addRequirements(m_command.getRequirements());
    }

    @Override
    public void execute() {
        m_command.execute();
    }

    @Override
    public boolean isFinished() {
        return m_command.isFinished();
    }

    @Override
    public void end(boolean interrupted) {
        m_command.end(interrupted);
        m_command = null;
    }
};