Round-trip flights spaced very far apart (or not taking the return at all) - any issues?

So the original plan was to book a one-way trip (Europe to the US - I'm moving back to the states for the foreseeable future) but they seem to be really expensive compared to round-trip tickets (over 1400€ vs. 600€ total). I'm considering booking the round-trip flight but scheduling the return flight to way down the line (say, end of the year) and decide later if I want to return for a vacation. Or just forget the return flight altogether.

1. how likely is getting blacklisted by the airline if I just not board my return flight? And:

2. how likely am I going to be questioned by customs and the airline if I show my booking with 6-12 months between my outward and return flights? (to be clear - I'm a dual citizen of both my origin and destination countries)