I'm going to take the assignment literally, and allow great one-year performers onto the roster. On any single Game Day:
Goaltenders:
Eddie Belfour - made UND 1.5 goals a game better than the rest of the WCHA. Before and after Belfour in the crease UND had great teams but didn't win titles, a Wins Above Replacement proxy.
Jon Casey - in the Wins Above Replacement calculation (if one existed) Casey would be way up there. Came within one late goal of leading the old WCHA in GAA 3 years in a row. Stood on his head to get UND to the Frozen Four in Lake Placid (1984) at RPI in the NCAA quarterfinal.
Karl Goehring - also "almost" led the WCHA in GAA three years in a row. The free wheeling teams in front of him could go full throttle knowing he was between the pipes.
Defenders:
James Patrick - skated powerfully and smoothly, and great on his stick. Scored the GWG at old Mariucci Arena on my birthday (3x3 with under 2 minutes to go in the game ... skated 200 feet with the puck and then fired a wrister (caught the Rodents by surprise since he rarely led the rush), then stayed on for the closing Rodent 6x3 PP). UND's first first-round draft choice (I think?) for a reason.
Matt Greene - fantasy pairing of Greene and Patrick? I can still hear guys squealing in front of the net as Greene laid the lumber on them ...
Troy Stecher - superb combination of speed, puck handling and positioning. Amazing guy to watch.
Craig Ludwig - blocked shots way before it became a science, and managed not to get injured (much). Intense on the ice ... back before face masks you could see the expression on their face and holy cow. Bench clearing brawl back in the 1980-81 ish range, and as players started to pair off, the opponent guys all pretended not to see Ludwig and went after someone else. Finally, someone took up the challenge, and Ludwig picked him up at arms length, shook him around like a rag doll, then dropped him on the ice without ever throwing a fist. Hilarous to watch.
Jake Sanderson - excellent two way defender. Marvel at his skating ability. Wizard with the one timers. Gotta be on the PP somewhere.
Ian Kidd - he can quarterback my power play anyday of the week and twice on Saturday. He definitely wanted to go to Detroit. (Google: UND Heavy Metal Hockey.)
Forwards:
(Note: UND with a history of great two-way 200-foot forwards. We're the only NCAA school with two different Selke Award winners, and 11 guys have gone on to get votes in the NHL's Selke Voting. Credit Gino first and Blais later for instilling that bit of culture.)
Greg Johnson centering Mark Taylor & Doug Smail: Johnson singlehandedly kept UND in many games and simply would not quit. Taylor and Smail both excellent two way players as well as my top two forwards on the PK.
Jonathon Toews centering, of course, TJ Oshie and Ryan Duncan: why mess with perfection? Toews scored a no-look goal from just above the red line after carrying the puck behind the net, in his first month as a 17 year old. Undersized TJ Oshie kept knocking over the other teams' much bigger defensemen. And Ryan Duncan scored 14 goals on 28 shots on his way to the Hobey.
Troy Murray centering Jeff Panzer and Dave Christian: let's call this a checking line, even though these guys all scored prolifically, and had a knack for scoring clutch goals. Panzer's great stick and quickness frustrated many opponent breakout patterns, Christian's instincts and skating meant he was always where the play was going to be. Troy Murray was a rock from day one.
Nick Schmaltz centering Tony Hrkac and Brock Boeser: I can't call this a fourth line, but what a PP unit this could be...
Extra skater: Zach Parise. Guy can fill in anywhere, and superb in clutch time.
I'll close by noting that UND has had so many great players that a second group of 21 could be assembled, that might just beat my team of 21 above. We've certainly had an amazing run of talent.