some ideas to tie single player exploration with a long term goal:
a special POI, one in every system, built as a "communication device", which, when unmounted, gives you a special part, like a memory data bank.
pick enough memory data bank, craft them and on the galaxy map locations of systems containing a special 'alien fortress world' appear (those would also be naturally discoverable, but rare, say, one in ten systems)
in these heavily armored fortress you'd get another component, like a...
on a longer PvE goal
a special POI, one in every system, built as a "communication device", which, when unmounted, gives you a special part, like a memory data bank.
pick enough memory data bank, craft them and on the galaxy map locations of systems containing a special 'alien fortress world' appear (those would also be naturally discoverable, but rare, say, one in ten systems)
in these heavily armored fortress you'd get another component, like a...
on a longer PvE goal