For each of the schools assumed to be candidates here are their most played teams (must have 40 games against or more). I have no clue how ECU has played for 85 years and not played any team more than 40 times so I included the 4 teams they have played more than 20 times. Because UCF (1996), UConn (2000) and USF (2001) is pathetic in that they've only even been Div I since the years identified I listed all teams they've played 10 or more times.

I included the first few columns included on the http://football.stassen.com/ site such as the frequency (gm), duration (first meeting), most recent game (last) and Win % to help gauge how much of a rivalry it might be. Some that may have only been played 30 times if they all have been in the last 30 years may qualify more than some that were played 60 times but not played in the last 20 years...

CSU								
	Opponent	First	Last	Win%	gm	W	L	T	
	Wyoming	        1903	2015	0.549	103	54	44	5	
	Utah	        1902	2014	0.287	80	22	56	2	
	Colorado	1902	2015	0.288	78	22	55	1	
	Utah State	1902	2015	0.528	72	37	33	2	
	Brigham Young	1922	2010	0.413	69	27	39	3	
	New Mexico	1935	2015	0.603	63	38	25		
	Denver	        1902	1960	0.555	55	28	22	5	
	Air Force	1957	2015	0.398	54	21	32	1	
									
BYU								
	Opponent	First	Last	Win%	gm	W	L	T	
	Utah	        1922	2015	0.367	90	31	55	4	
	Utah State	1922	2015	0.571	85	47	35	3	
	Wyoming	        1922	2010	0.591	77	44	30	3	
	Colorado State	1922	2010	0.587	69	39	27	3	
	New Mexico	1951	2010	0.758	60	45	14	1	
									
Memphis								
	Opponent	First	Last	Win%	gm	W	L	T	
	So. Miss	1960	2012	0.387	53	20	32	1	
	Mississippi	1960	2015	0.261	46	11	33	2	
	Louisville	1961	2013	0.415	41	17	24		
									
Cincy								
	Opponent	First	Last	Win%	gm	W	L	T	
	Miami (Ohio)	1954	2015	0.46	62	28	33	1	
	Louisville	1966	2013	0.5	45	22	22	1	
									
Houston								
	Opponent	First	Last	Win%	gm	W	L	T	
	Rice	        1971	2013	0.725	40	29	11		
	Tulsa	        1950	2015	0.55	40	22	18		
									
ECU								
	Opponent	First	Last	Win%	gm	W	L	T	
	So. Miss	1966	2013	0.316	38	12	26		
	NC State	1970	2013	0.429	28	12	16		
	Memphis	        1990	2012	0.714	21	15	6		
	West Virginia	1970	2009	0.143	21	3	18		
									
UConn								
	Opponent	First	Last	Win%	gm	W	L	T	
	Rutgers	        1979	2013	0.462	13	6	7		
	South Florida	2000	2015	0.385	13	5	8		
	Cincinnati	2001	2015	0.167	12	2	10		
	Buffalo	        2000	2013	0.818	11	9	2		
	Louisville	2000	2013	0.4	10	4	6		
	Temple	        2001	2015	0.4	10	4	6		
									
USF								
	Opponent	First	Last	Win%	gm	W	L	T	
	Cincinnati	2003	2015	0.385	13	5	8		
	Connecticut	2000	2015	0.615	13	8	5		
	Louisville	2003	2013	0.455	11	5	6		
	Pittsburgh	2001	2012	0.3	10	3	7		
									
UCF								
	Opponent	First	Last	Win%	gm	W	L	T	
	East Carolina	1996	2015	0.364	11	4	7		
	Marshall	2002	2012	0.727	11	8	3		
	AL-Birmingham	1996	2012	0.8	10	8	2