I really hope the final release gets some more Singleplayer missions.
But even without, granting players high scores (Final fleet score multiplied by honor) for taking out the SP missions in a particularly inspired way would up the SP replay value a good deal.
You can already see the tooltip that lists the most honor you’ve had when completing a mission by hovering over the difficulty icon for that particular mission. You could try and best that til you’re blue in the face it you would like 
But honor is only a measure of how few ships you brought to the battle. Bringing a minimal fleet and just barely making it counts for more than a reasonably sized but excellently balanced fleet that solidly trounces the enemy with minimal casualties. Multiplying the honor by the final fleet score would be a better measure of skill, IMO.
I doubt the Galactic Senate would agree, “DO MORE WITH LESS!!”.
I get your point and do agree
You could have something along the lines of score = honor - (point value of ships lost) to reward careful planning
I like the idea of high score lists But why stop at one list when you can have several? Things to track for each mission would be:
Highest Honour
Quickest Time
Percentage Fleet (Tiebreak by Honour)
Some kind of score that balances the three above - some kind of (Honour Saved-Honour Lost)/Time
Maybe also track the three things plus number of fighters/frigs/cruisers in the list (and have a World Ranking!) (and a bit of text as name/description) to remind you of how you scored that ranking (maybe even save all the ships and the deployment - not sure how much space that would take…)
You could even give medals/trophies/etc. for making various levels (Gold/Silver/Bronze)
There could also be special challenge achievements for each mission (easy, medium and hard ones):
No fighters (or no frigates, or no cruisers) (or Only frigates/Only cruisers. Currently only fighters is a bit tough due to low fighter limits…)
No point defense
No armour (or no shields. Or no defense at all?!)
No (particular weapon type/particular module type)
Only lasers (or beams, or missiles - focus on one type of weapon)
Conditions could be complicated, like: 100% fleet at the end AND no cruisers.
This would encourage people to build/try different weapon combos - could also be made part of the challenge system.