In order of priority for me:
- Energy efficiency
- Sustainability
- Interesting/Fun Engineering/Science
- - - - - - - -
As for interstellar travel ..
I don't expect any society technologically advanced enough to be able to do it .. to have any interest in doing it .. in the classical sense anyway .. I only see 2 possible exceptions .. and in any event absolutely no interest in us at all.
#1> Whole Society in Generation Ships:
If your species / society survives for enough billions of years .. in order to continue to survive you would eventually have to turn to at least generation ships .. but these don't need to 'travel' any faster than just slowly getting to the next resource extraction site every couple billion years or so ... sit at the new resource collection site for a few billions years before eventually taking a few billion more years to travel to the next ... etc .. no FTL space ships zipping around is ever needed nor desired.
#2> Dark Energy Drive:
The gist of the theory of dark energy is that empty space itself contains a form of energy and applies a negative pressure (ie a push/expansion of the space-time itself) .. it's one explanation for the accelerating expansion of the observable universe .. Relativity's Speed of Light limit doesn't apply to the movement (expansion) of space-time itself .. Instead of a 'Warp Drive' which has to spend very large amounts of energy to bend/warp the existing space-time .. a Dark Energy Drive .. consumes/converts the space in front of it (as an energy source) .. and in so doing there is then less space (ie distance) in front of that object ... In this way traveling long interstellar distance doesn't 'cost' you energy at all .. it gathers / extracts energy ... the further you travel the more energy you can gather .. the faster you go the faster you can gather more energy .. Dark Energy is estimated to be the largest energy source in all of the observable universe.