Due to the number of current POVs (20!) and the number of POVs in the immediate area (Barristan/Tyrion/Victarion and pontentially Dany), many readers consider Barristan's days numbered. While I could see his plotline going a few ways (dying, staying with Dany, switching to Young Griff), I thought it was worth noting that if Barristan does indeed die during the Battle of Fire who would take over as battle commander... and the answer is... The Widower

The Widower: Leader if Barristan Dies

Note: I am NOT arguing what happens to Barristan in the post, just discussing the new commander should Barristan fall (and/or turn his cloak)

While discussing the battle plan for the Battle of Fire with his war council, Barristan states this when asked what should happen if he dies:

The Widower's horse sidled to his left. "And if your horn falls silent, ser knight? If you and these green boys of yours are cut down?" It was a fair question. Ser Barristan meant to be the first through the Yunkish lines. He might well be the first to die. It often worked that way. "If I fall, command is yours. After you, Jokin. Then Grey Worm." -TWOW, Barristan I

If Barristan dies (hopefully none of "Barry's Lads" follow him) he has appointed The Widower (current leader of the Stormcrows) to take over.

Background

As the Stormcrows assemble for the Battle of Fire, we get a description of the Widower:

The Stormcrows had assembled beneath the merchant's arcade fronting on the southern side of the square, where the arches gave them some protection from the dead men. Jokin's archers were fitting strings to their bows as Ser Barristan rode by. The Widower sat grim-faced astride a gaunt grey horse, with his shield upon his arm and his spiked battle-axe in hand. A fan of black feathers sprouted from one temple of his iron halfhelm. The boy beside him" "was clutching the company's banner: a dozen ragged black streamers on a tall staff, topped by a carved wooden crow. -TWOW, Barristan I

Joint Command of the Stormcrows w/Jokin:

The Stormcrows were represented by two seasoned sellswords, an archer named Jokin and the scarred and sour axeman known simply as the Widower. The two of them had assumed joint command of the company in the absence of Daario Naharis. -ADWD, The Queen's Hand

although it seems he does have a little more power than Jokin:

THE STORMCROWS, five hundred horse-riders, sworn to Queen Daenerys,

- DAARIO NAHARIS, captain and commander,

- THE WIDOWER, his second-in-command,

- JOKIN, commander of the company archers. -ADWD, Appendix

The Stormcrows

When we first meet them the Stormcrows are led by three characters (Daario and Sallor the Bald/Prendahl na Gehzen) before Daario kills the other two.

Daario then has sole control of the Stormcrows until he is given to the Slavers as a hostage. This is when the Widower assumes control:

With Daario a hostage of the Slaver's, we see Jokin and the Widower speaking for the Stormcrows:

Barristan Selmy could feel their eyes upon him. The captains and commanders advanced to meet him. Jokin and the Widower for the Stormcrows, ringmail clinking under faded cloaks; Grey Worm, Sure Spear, and Dogkiller for the Unsullied, in spiked bronze caps and quilted armor; Rommo for the Dothraki; Camarron, Goghor, and the Spotted Cat for the pit fighters.

and they are charged with toppling the Harridan (trebuchet):

"You know our plan of attack," the white knight said when the captains were gathered around him. "We will hit them first with our horse, as soon as the gate is opened. Ride hard and fast, straight at the slave soldiers. When the legions form up, sweep around them. Take them from behind or from the flan, but do not try their spears. Remember your objectives."

"The trebuchet," said the Widower. "The one the Yunkai'i call Harridan. Take it, topple it, or burn it."

Jokin nodded. "Feather as many of their nobles as we can -TWOW, Barristan I

but they do ask Barristan what to do if they see Daario:

And if we come upon the captain?" asked the Widower.

Daario Naharis. "Give him a sword and follow him." Though Barristan Selmy had little love and less trust for the queen's paramour, he did not doubt his courage, nor his skill at arms. And if he should die heroically in battle, so much the better. "If there are no further questions, go back to your men and say a prayer to whatever god you believe in. Dawn will be on us soon."

"A red dawn," said Jokin of the Stormcrows.

A dragon dawn, thought Ser Barristan. -TWOW, Barristan I

Barristan had earlier worried about them taking matters into their own hands wrt Daario:

Daario is vain and rash, but he is dear to Her Grace. He must be rescued, before his Stormcrows decide to take matters into their own hands. It can be done. I once brought the queen's father safely out of Duskendale, where he was being held captive by a rebel lord, but …" -ADWD, The Kingbreaker

It should be noted that another sellsword company (The Second Sons led by Brown Ben Plumm) are potentially going to free the hostages (including Daario).

He is then mentioned (briefly) again in this fan summary of TWOW, Barristan II

His gut feels twisted from nervousness as he rides through the gates. He knows that the feeling will go away when time slows down in the chaos of battle. Dany’s horse is easily outpacing the lads and the rest of the cavalry; Barristan is pleased because he intends to outrun the Widower and strike the first blow. The Yunkai’i are totally unprepared and Barristan closes in on the Harridan, the largest of the trebuchets. The stormcrows take up the cry, “Daario!” and “Stormcrows, fly!” Barristan thinks that he will never again doubt the valor of sellswords. -TWOW, Barristan II (fan summary)

Again I want to note that this post was about who would take over if Barristan were to die, not discuss whether he lives/survives/turns his cloak. There are great posts about all of these options, but that wasn't my goal here.

TLDR: If Barristan dies in the Battle of Fire then the command of the Meereenese forces goes to the Widower, a sour grim-faced axeman who is currently in (joint) command of the Stormcrows.