I've been traveling around from nebula to nebula, just to go see them. I find I hit plenty of undiscovered systems. I also find that many of the systems that have already been discovered are incomplete (bodies not scanned), and many also have bodies not mapped that are worth mapping (landable high metal content, rocky/icy bodies)**.
How much time you're willing to spend in a system can impact 'discovery'... it appears many Cmdrs cruise though each system looking for specifics (ELWs and WWs), and spend little time on anything else. So just because you've run across yet another system already discovered, doesn't mean there's nothing there for you. If you enter a system and honk and it says '25 bodies', but the map only shows 20 - then get to work, they're yours.
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I'm finding that some of the landable icy bodies have POIs even though the FSS scanner says no. The POIs show up once the body has been mapped with DSS. The POIs are typically Geo. Occasionally, I'll find this also with landable high metal content bodies, and rocky ice bodies. I don't know if other Cmdrs find this as well, if it's a bug or what, but it has earned me discovery and Codex credits.