Of the ACC teams UVA makes the most sense for SU as being a rival. Location is close enough, similar FB, BBall, Lax. Problem is they have UNC and even Duke. And they are Southern not Northeastern.
Of the non ACC teams MD makes the most sense. They really do not have anyone either. They used to play UVA (see above) and WV yearly but those are more like our BC and Pitt games. Traditional rivals but not RIVALS. Our football programs are similar calibre, we both care about BBall, and have Top 5 Lax programs. On top of that we share a similar student body pool.
The 2nd best OOC choice unfortunately is probably Rutgers. Historically FB is not even close. But going forward they are in the same ball park. Their BBall has improved since joining the B1G. They do play Lax and have improved there as well.
If we are playing the long game I suppose Nova could become THE rival. BBall is obvious. They do play Lax. FB would need to jump up to FBS and then eventually build up to where we are both in the same conference. However, that would likely mean SU FB gets left behind, so this choice would be depressing. Too bad Nova didn't jump to Big East FB when they had the chance.