Comets are already included in the Stellar Forge background simulation. They arent visually represented in the game though, but as stated by Mr. Braben, they will get a visual appearance for sure. Only when is the question.
An asteroid deflection mission? This isnt Orbiter or KSP... There is the capital class solution for minor threats like that...
Edit:
Stellar Forge also checks if celestial bodies attract each other. That means, that systems can merge or rip apart, as far as i know. So we might already have rogue planets, but the problem is, that the FSD can only target large mass objects like suns... So we will never encounter one, unless you know of one and go there by Supercruise...