The theme is revealed at 21A: With 28-Across, a late, great entertainer (Beverly/Sills). Other theme answers are:
17A: 1966 Lincoln Center role for 21-/28-Across (Cleopatra).
48A: 1970 Covent Garden title role for 21-28-Across (Lucia).
58A: Childhood nickname of 21-/28-Across (Bubbles).
62A: 1955 "Die Fledermaus" debut role for 21-/28-Across (Rosalinde).
11D: "La Traviata" role for 21-/28-Across (Violetta).
29D: With 39-Across, 21-/28-Across, for one (lyric/soprano).
38D: Stage wear for 21-/28-Across (costumes).
That is so much more than I knew about Beverly Sills. I knew her nickname...I had a penpal when I was in elementary school named Beverly, and she was also called Bubbles. I was also able to guess costumes and lyric soprano. This article tells about a side of Beverly Sills that many didn't know, describing her as "special mother to the special children."
I'll be the first to admit I had to use Dogpile to get several of these. While I had letters in place from crossings, some of the names were just too vague for me.
Some of my favorite clues and/or answers:
23A: Insignificant (nominal). "The line is busy. For ONLY ninety-five cents..." As if I'm willing to pay what they consider a nominal fee to have Qwest redial the number for me.
61A: Stretches out? (comas). Not that it's funny, but the clue is certainly clever.
1D: Mysteries (arcana).
8D: Conger, e.g. (sea eel). EAEE just looked wrong. Anyway, I'm bettin' it's a pretty ugly thing. We're in luck...no photo available at that site.
9D: Unlikely candidate for Mr. Right (creep). Too funny...how many of us fell for Mr. Creep anyway?
41D: Linguist Chomsky (Noam). I have always liked the name.
49D: Requirement to buy on eBay (user ID). I try to stay away from eBay. In a matter of two months, I spent more money on Hummel figurines than I care to admit...or more than I want Don to find out if he reads this.
52D: Cause to burn (kindle). I had ignite at first...a good wrong answer. I started a fire in the microwave at work yesterday, heating up a pizza in the box. The box was microwave safe, but I had absentmindedly put a sticker on the box, and it wasn't. Fortunately, I was able to put it out before any damage occurred...but it did stink.
Things I didn't know:
27A: "As I Lay Dying" father (Anse). That looks like something I should remember...like maybe it will come back.
68A: Ruhr Valley city (Essen).
3D: Starting lineups (A-teams). I've fallen for this one before. I had at bats. I'm not even sure I could call that a good wrong answer.
37D: D-backs, on a scoreboard (ARI). Would someone please explain this? I'm sure I'm not the only one who doesn't get it.
We have similar answers at 13D: Expert finish? (ise) and 34D: Believer: Suffix (ist), and 55D: "The Wild Duck" playwright (Ibsen) crossing Essen (see above).
We also have identical clues at 7D: Balkan native (Serb) and 43A: Balkan native (Croat). I thought I was seeing things...maybe getting a little 33A: Scatterbrained (ditsy).
Another playground retort rears its ugly head at 35A: Retort to "Not so!" (is too). Making another appearance, to the dismay of many, is 51D: More ludicrous (inaner). And there will be some offended by 65A: Really big (obese).
That's it for tonight. Here's the grid...

...and I'll see you tomorrow.
Linda G