Should Ohtani pitch, hit or both in 2020?

On Wednesday, the Angels announced that Shohei Ohtani's injured right elbow would likely require Tommy John surgery, knocking him off the mound for the remainder of this year and all of 2019. For most players, that would be the end of it. They'd exit the public eye to spend a year-plus rehabbing, and you wouldn't think much about them until they were nearing a return. We're not talking much about Garrett Richards or Johnny Cueto right now, are we?
But Ohtani, needless to say, is not "most players." Hours after the elbow news surfaced, he went out and hit two homers as part of a four-hit night in a 9-3 win over Texas. He hit another on Friday in Chicago, and tripled, singled and walked on Saturday. In six September games, he's got 11 hits. He's proven without a doubt that he can be a high-quality Major League hitter, and due to that, we'll still see him taking swings for the rest of the year and next season, as well. 
He's been so good as a hitter, in fact, that there's at least now a question about what is the best way to utilizing him going forward. Should he hit full-time? Should he stay on the mound, but with his pitching managed differently than it was this year? Let's run through a few of the options -- but first, let's remind you just how good he's been this year.
Ohtani has exceeded any reasonable expectations
Remember when people were worried if Ohtani could hit in the big leagues?
Ohtani's line right now is .294/.375/.595, with 19 home runs. While his 297 plate appearances aren't enough to qualify for leaderboards, if they were, that would be the fifth-best line in the game, behind four Most Valuable Player candidates. His slugging percentage would be fourth best, in part because he has a Top 10 exit velocity on fly balls and liners, tied with Aaron Judge. His hitting season, if he'd had enough playing time to qualify, would be the second-best year by a rookie Angels batter behind only Mike Trout's 2012; it would be one of the six best rookie hitting years in the past 50 seasons. Remember: This is all only as a hitter.

On the pitching side, his strikeout rate (29.9 percent) is the same as what Blake Snell has put up. If you look at the advanced Statcast™quality-of-contact metrics, Ohtani has basically been James Paxton as a pitcher combined Anthony Rizzo or Francisco Lindor as a hitter. He's been absolutely incredible, aside from health.
If we look at FanGraphs' Wins Above Replacement, Ohtani has been worth 2.7 WAR as a hitter and 1.0 more as a pitcher. If he'd stayed healthy the entire year, and maintained this performance level, he might have put up about 6 WAR, or about what All-Stars like Matt Chapman and Manny Machado have compiled to this point.
That's very, very good. It might also be about the limit to what he can do as a hybrid player. Remember, when he was starting regularly, he required days off before and after his pitching appearances. In May, for example, the Angels played 29 games. Here's how his month broke down:
SP: 4 times
DH: 15 times
PH: 2 times
Did not play: 8 times
Ohtani started four of them on the mound, was the DH 15 times, and pinch-hit twice more. In other words, he missed eight games entirely, didn't bat in four others and took just a single at-bat twice. So in 45 percent of the games the Angels played in May, he wasn't a full-time hitter. For someone who's hitting like a superstar slugger, that's a lot of time to not be hitting.
Plus, because he often received extra rest between starts, he wasn't a full-time pitcher, either. Ohtani threw 25 innings in May -- good ones, with a 2.16 ERA -- but 111 pitchers threw more innings that month.
So, that's where we're starting. Ohtani, even when healthy, was neither a full-time pitcher nor a full-time hitter in 2018, and might have been on a 6 WAR pace. We know he won't be pitching in 2019. What should or could he be in 2020? 

He could be a DH in 2019 and return to the rotation in 2020
This is more or less what he did this year, and so it seems like that's what will probably happen again when he's ready to return to the mound in 2020. He'll DH a few times a week and start once a week. It's not all that interesting to ponder how this might look, because we already know what it looks like.
For 2019, however, it's an intriguing question, because his inability to pitch means that he should be available to DH more. (No, he won't play the field, not with the need to protect that recovering throwing arm.) We were probably looking at about 400 plate appearances this year, around his pitching, if he'd been healthy. Without the days off around his starts in 2019 -- and assuming he's ready for Opening Day -- we might be seeing north of 600 plate appearances. That's a huge difference.

That's both a benefit to the Angels and a problem: How do they find DH time for both Ohtani and the soon-to-be 39-year-old Albert Pujols, once one of the greatest hitters in the game, but who has hit only .243/.287/.397 over the last two years? Pujols just had his season end early due to knee surgery, and his ability to play first base in 2019 is very much in question. Any playing time given to Pujols over Ohtani is a big net negative for the Angels' offense.
He could give up pitching and focus on hitting: DH version
This seems unlikely, because he was so good as a pitcher, and because he chose to sign with the Angels in part because they agreed to allow him to be a two-way player. But let's play this out for a minute, anyway, because it's worth asking the question. Across 2017, '18 and '19, Ohtani is going to have thrown all of 77 innings. Durability is a concern. Can the 2020 Angels count on a large number of innings? If not, should he just hit every day? Put another way: Would you rather have the extra 200 or so plate appearances, or the unknown number of innings he'd contribute?
Let's assume that Ohtani was an everyday designated hitter, and that his 2018 batting performance is his true talent level. (That's perhaps not true, but it's what we have to go on right now.) If we look back on the 21st century, finding other designated hitters with similar batting lines to Ohtani's, we get an average of about 5 WAR, looking at years like 2000 Frank Thomas, 2005 Travis Hafner, 2005 Jason Giambi, 2014 Victor Martinez, and 2016 David Ortiz.

That's one Hall of Famer and another who belongs there, which is why it's risky to assume Ohtani could hit that well for a full season. But if he did: 6 WAR seems to be near the limit of the value he could provide this way, unless he upped his hitting even more.
He could give up pitching and focus on hitting: OF version
What if he played the outfield in 2020? He's got the arm for it, obviously. He's got the speed; he's the third-fastest Angel this year. He's got experience, having played 62 games there in Japan, though admittedly none since '14. The Angels wouldn't have to pick up Kole Calhoun's 2020 option, opening up right field.
Depending on his defensive skill, the range there with the current version of his bat would be roughly between 5 and 7 WAR, or within the margin of error of roughly what he was going to offer this year anyway. (You'd have to be elite on the bases and in the field, like Trout in 2012, or a disaster at both, like Manny Ramirez in 2006, to over- or under-perform this significantly.) It seems unlikely this would happen, but it should remain as an option.
He could be a DH in 2019, and be an DH/OF/'opener' in '20
OK, let's get weird. There's more than one way to be a pitcher in 2018, as the Rays have proven very well by utilizing the "opener" (along with Snell) to have one of the year's better pitching staffs.
What if instead of Ohtani throwing seven innings once a week as a starter, he started two times per week and threw two or three innings apiece? There's so much uncertainty with this; would it be better for his arm (fewer long outings) or worse (more outings)? Might it improve the fortunes of his teammates? For example, if Nick Tropeano or Jaime Barría entered to face the bottom of a lineup rather than the top, would that benefit them? If he started as an opener, would they maneuver to keep his bat in the lineup afterwards? Could this be handled to allow him to face mostly righty pitching? (Some have suggested he DH and pitch out the bullpen, but the in-game preparation required of relievers might make this more difficult, so we're going with the opener plan instead.)
This would require a great deal of strategy, in terms of when he played, and where, but it might also allow for more plate appearances, if the shorter outings meant he did not require full days off before and after his starts, as he does now. Think of this version of Ohtani as one that is less a "hybrid hitter/pitcher," and more of one that's "a hitter who pitches sometimes," splitting hairs though that might be.
Now: WAR doesn't capture the value of having one roster spot being available for multiple uses. No metric can really account for everything Ohtani can bring, because there really hasn't been a player like Ohtani. We know he's going to hit in 2019, and that he's not going to pitch. Depending on how well he mashes next season, we might have an entirely different take on this question headed into 2020.