Some great choices, here are a few more:
LB Billy Cannon Jr.- 1984 1st round draft pick broke his neck halfway through his rookie year, this combined with the Sherrard injuries led the way to a talent gap and subsequent losing seasons
CB Kevin Smith - might have been the best CB ever drafted by the Cowboys but a foot injury ruined his career. I never saw another CB do a better job on covering Jerry Rice.
TE Todd Christensen - drafted as a FB, changed to TE, got into a contract dispute with Tex Shramm, This time Tex was wrong, Christiansen went on to a Pro Bowl career with the Raiders. Wait, it gets worse. Because he was no longer on the team, the Cowboys needed another TE, so in the 3rd round the Cowboys bypassed the BPA, a quarterback, and took TE Doug Cosby. Cosby had a nice career but nothing like the guy who the Cowboys passed over. The QB's name was Joe Montana. So, yeah, it would have been great, Montana a Cowboy, throwing to his TE, Todd Christiensen
Carl Lewis, as in Olympic Gold Medal hurldler, Carl Lewis drafted in the 12th round of 1984. Lewis decided against an NFL career.....such a shame. He could have been great.
DT Scott Appleton - the Cowboys drafted the big DT from Texas in the 1st round, 4th overall pick but he was stolen by the AFL when the Houston Oilers acquired him. Appleton was Staubach's nemesis during the college "championship" game featuring #1 ranked Texas against #2 ranked Navy. He would have been part of a devastating duo, him and Bob Lilly.
G Billy Shaw - Bob Lilly wasn't the only Hall of Famer taken by the Cowboys in their inaugural draft. Shaw was selected in the 14th round but, again, he got snatched by the AFL team Buffalo Bills.