In the spirit of some of my more recent posts about Dany's potential stops on the way back to Westeros (The Demon Road, Volantis and Pentos), some other major plot points exist in Slaver's Bay that at least need to be addressed if not solved, before Dany can head back.

the Sons of the Harpy plot in their pyramids, the pale mare rides down slave and lord alike, our friends from the Yellow City gaze out to sea, and somewhere in the grasslands a dragon nibbles the tender flesh of Daenerys Targaryen. Who rules Meereen tonight? Who will rule it on the morrow?" -ADWD, The Spurned Suitor

The Open Plotlines in Slaver's Bay (and Beyond)

Note: Dany is currently on the Dothraki Sea with Drogon during the Battle of Fire. Therefore if/when she arrives back in Slaver's Bay, you can expect the status quo to be very different. There is a bit of ambiguity between the two plotlines and Dany and the horselords could arrive at anytime, but due to GRRM's goal of including the Battle of Fire in ADWD originally and now the opening of TWOW, I think its likely they arrive sometime after the battle.

If interested: The Dothraki Sea Plotline in TWOW & Battle of Fire: Post Battle Power Vacuum

Meereen Issues

Pale Mare

Also known as the bloody flux (dysentery), it is extremely deadly:

Soon after came the sickness, a bloody flux that killed three men of every four -ADWD, Daenerys V

and:

I have known the bloody flux to destroy whole armies when left to spread unchecked. -ADWD, Daenerys V

Harpy/Sons of the Harpy

After Hizdahr is arrested, the Sons of the Harpy resume their shadow war.

Night 1: 3 deaths

Night 2: 9 deaths

Night 3: 29 (+1 more during the day) deaths

According to Galazza Galare, the killings will stop when Hizdahr is freed.

The identity of the Harpy is often debated and will likely be solved before the plot leaves Meereen (or flashing back) as well. Numerous great theories exist ranging from Reznak to Hizdahr to the Shavepate or the Green Grace.

Freedmen

While some of the freedmen have started to sell themselves back into slavery, many more will follow Dany. I am assuming any of those who do will be forced to march the Demon Road.

South of the ordered realm of stakes, pits, drills, and bathing eunuchs lay the encampments of her freedmen, a far noisier and more chaotic place. Dany had armed the former slaves as best she could with weapons from Astapor and Yunkai, and Ser Jorah had organized the fighting men into four strong companies, -ASOS, Daenerys IV

Slaver's Alliance

• Yunkai, New Ghis, Volantis, Qarth, Astapor Tolos, Mantarys,
• Hostages (Hero, Daario, Jhogo)

Sellsword Issues

• Brown Ben Plumm + Second Sons (including Tyrion/Jorah and "Penny")

If interested: The Battle of Fire: The Second Sons

Note: Brown Ben has "two drops" of dragon blood

• Tattered Prince + The Windblown

Magical Issues

• Quaithe vs. Moqorro vs. Marwyn
• Dragonhorn

Dragonbinder: Claiming the Horn

• Dusky Woman

If interested: The Battle for Control of Victarion

If interested: The Sowing: Finding New Dragonriders

Return Issues

Route

• The Path Back to Westeros: The Demon Road
• Sailing around the Smoking Sea

Division of Troops/Fleets

Which troops march and which take the ships, etc.

Returning to Westeros: Dany's Fleet

Stops

Including several cities/ruins along the Demon Road (Mantarys, Tolos, etc.) there will likely be stops in the following places:

• Volantis

The Path Back to Westeros: Volantis

• Pentos

The Path Back to Westeros: Conflict in/over Pentos

Perfumed Seneschal

While the others in this passage are likely known, this identity remains up for grabs and could be anyone from Marwyn to Varys to someone who was about the Stinky Steward, or even the "Citadel's Man:

"No. Hear me, Daenerys Targaryen. The glass candles are burning. Soon comes the pale mare, and after her the others. Kraken and dark flame, lion and griffin, the sun's son and the mummer's dragon. Trust none of them. Remember the Undying. Beware the perfumed seneschal." -ADWD, Daenerys II

Arrival of The Citadel's Man

As this is just a thought Marwyn has as he plans to board the Cinnamon Wind and take her to slaver's bay, its no guarantee one exists, but the idea of the Citadel sending someone after Daenerys makes sense:

"What will you do?" asked Alleras, the Sphinx.

"Get myself to Slaver's Bay, in Aemon's place. The swan ship that delivered Slayer should serve my needs well enough. The grey sheep will send their man on a galley, I don't doubt. With fair winds I should reach her first."

Loyalty Issues

Barristan

If Daenerys/GRRM's solution to speed up this plotline is Daenerys burning cities, does Barristan's loyalty shake? Plenty of great theories about him going over to Young Griff exist.

He has already created some strife by promising Pentos to the Tattered Prince.

Jorah

Wants to return to Daenerys (brought her Tyrion) but is sworn to the Second Sons and has sworn to vouch for Brown Ben Plumm with Dany.

Tyrion

Tyrion and Dany are expected to spend most of TWOW apart according to GRRM, which lends credence to Dany's plotline in the Dothraki Sea taking longer than originally thought, Tyrion heading back to Westeros with someone else or they being in the same area but not encountering for some time.

“Well, Tyrion and Dany will intersect, in a way, but for much of the book they’re still apart,” he says. “They both have quite large roles to play here. Tyrion has decided that he actually would like to live, for one thing, which he wasn’t entirely sure of during the last book, and he’s now working toward that end—if he can survive the battle that’s breaking out all around him. And Dany has embraced her heritage as a Targaryen and embraced the Targaryen words. So they’re both coming home.”

Dornish

Gerris Drinkwater and Archbald Yronwood have been spared by Barristand and due to Arch's hatred of ships will likely march the Demon Road. They are heading back with Quentyn's bones and sad news. This likely pushes Dorne into an alliance with Young Griff.

Ironborn Alliance

Victarion/Euron are in Slaver's Bay to a)marry Dany and b) get a dragon. They aren't going to aid Dany without those.

Even after all of that there are potential issues in the Stepstones.

TLDR: A list of the major plot points, issues that need to be resolved after the Battle of Fire before the plotline can move forward completely to Dany's return. The resolution of these plotlines will take some time, and while I can see a couple being solved via just "fire and blood" there are far too many open (some just created) to be solved like that. And while the Tyrion/Dany quote about being "apart" is a bit ambiguous, it does seem to indicate that not everyone is going to come together nicely under Dany and divide up and head back.