Lets see......$15K? One bike will never keep anyone 100% happy. The spectrum of riding is just too diverse. If its good off road, it'll be uncomfortable/inefficient on long road trips or vice versa.
Used 2nd Gen FJR1300 for long road trips : $5K-$6K
Used 2nd Gen SV650 for streets around town and for sporty riding: $3K
Used KTM 690 or a WR450 for dual sporting....maybe something smaller for more trail oriented riding.
So far I've got my SV and come spring will probably pick up a cheap dirt bike and possibly a 2013 FJR.