I think we have to be careful about going overboard about what a 3-4 NG should be. First of all, there is a significant difference between a 1 gap 3-4 and a 2-gap 3-4. One requires more of a penetrator in the middle, while the other requires an anchor who can stalemate the A gap or the "1-2 hole" to either side of the center. Lots of teams have been successful running a 1 gap 3-4 defense, without a 340lb Louis Nix in the middle. Remember that Sean Cwynar and Kona Schwenke were both 300lb players and they played high quality snaps in a reserve role at NG... Guys like Darnell Ewell and Myron Tagovaiola-Amosa (would could be well over 300lbs by next year if you wanted him to be) are plenty big enough and strong enough to to be a 1 gap NG in a 3-4 like at Stanford or Alabama. If you are going to play the NFL style 2-gap, 3-4, like under Diaco, ND would have significant roster issues right now... Not only does that defense call for a big NG, but both your DE's are also going to be essentually athletic DT's, like Tuitt, Springmann and and Lewis-Moore were.
ND could has the interior DL to run a 1 gap 3-4 scheme in the future with Darnell, Myron and Ja'mion. Sam Taimani would be a great fit there as well. They alsonhave excellent canditates to play the hybrid position on the outside in Daelin Hayes, Julian Okwara, Jamir Jones, Kofi Wardlow and Jack Lamb. What they lack are 3-4 defensive ends after Jay Hayes graduates. Outside of Jayson Ademilola, there just isn't a lot of those guys in the pipeline.
I just think it's important that we properly differentiate between schemes and note key differences... It's also important to note that even 3-4 defenses tend to be playing with some smaller, more agile players in their back 8 these days as they adapt to spread offense and the plethora of passing offenses that have become the norm. Your linebackers have to be exceptionally mobile in the game of football today.
