Exploration has proved tricky for FD to missionize, either in regular missions or in CGs. Partly because, as mentioned above, the game doesn't track many of the features you would logically need, and partly because missions and CGs are always time-based, and exploration - proper exploration - takes longer than a typical mission or CG timeframe allows.
Eg. a "logical" exploration mission would be "We're planning on building a new deep-space colony in the Blau Aec sector. Fly to the Blau Aec sector and find at least five Earth-like planets". That's kind of vague; Blau Aec might or might not have any ELWs, and finding that many in any kind of reasonable timeframe will be tricky. I got lucky when I visited Blau Aec and found five ELWs in 24 hours, but it usually takes me two weeks to find that many. And Blau Aec is 12,000 LYs away; it would take me three weeks to travel to Blau Aec, and three back again. Missions don't normally have a seven-week time window.
Or how about "We've received some ambiguous readings on the Blau Aec PH-C d3 system, we want you to go there, scan the entire system, and report back with your findings". Here, besides the timeframe for getting there and back again, there also has to be some way for the mission generator to figure out that there's something in that system that's worth sending someone 12,000 LYs to re-scan. In this case, the explorer doing the mission will be saying to themselves, "it's just an Earth-like planet. I passed through five Earth-like-planet systems on my way here; what makes this one so special?".
Finally, what kind of rewards are going to be offered, to tempt explorers into constraining their normal open-sandbox activity for a mission-giver, perhaps for a couple of months? Credits? Explorers don't care about credits, or they wouldn't be explorers. Likewise influence, reputation and rank. The only thing I can think of that might tempt an explorer is the issue of an otherwise-unobtainable sector permit on completing the mission. And then, it would have to be a hand-created quest, not a procedurally-generated mission.
