PAK'll never open with Babar. Thanks God that he didn't fail at 3 in his first few innings, otherwise No. 5 was destiny.
Ideally, he should open, because -
1. He is poor against spin - poor, not weak. But, very good against pace - opening allows him time in the middle before he takes on spinners
2. He is a sweet timer with very good placing, and he plays on grass - with 9 men inside, he has better chance to punish couple of balls selectively in gaps rather than swinging around after every ball
3. PAK has a good stock of lefti openers now - FZ, Amin, Sami, Shan (he is scoring crazy, has to count), Imam ..... even SK can come back in a years or two time. Making BA opener means, the right-hand end could be sealed for a decade
4. Babar has a tendency to slow down in his 4th quarter (75-100), which often cuts path between 40-50 overs. Making him opener allows him to reach 100 in between 35-38th overs. It sounds like the purpose is to serve him, but I don't see any problem if it's a win-win for both him & team. Best batsman of the team should get the opportunity of playing most number of balls.
5. He is the only top order who can play quality boundary shots without putting extra "labor", MoHa was like that, but his defense was too poor against new ball - this guy is much better.
Some batting buzurg must have scared him regarding the swing & seem of 320 par ODI games these days, hence he has dropped down even in domestics. From U13 to early domestic career, he was opener and he had very good List A numbers.
1-3. Babar, FZ, Umar+

,
4-6. Hari, Maqsood, Talat
7-8. Shadab, Fahim
10-11. Amir, Hasan
*Amin or Saud or a 3rd pacer - depending on condition (In case of Amin/Saud, they play at 3 pushing everyone one slot down)
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14. New Pacer - preferably a right armer, but not essential - either of the 2 U19 kid is extremely impressive (My pick is Arshal - may be both as 3rd & 4th pacer - Raees, Shinwari, JK doesn't excite me much)
15. Raza Hasan (or whoever is best finger spinner, may be Aga Salman as well, because Hari & Saud can bowl SLAO, one offie should be in squad),
16. Ruhail Nazir+
Exciting, excitingly good squad for 2019 - pick a Captain, my choice Amir, but don't mind FZ, Hasan, Babar or even Shadab
Hope, Sarfraz fans are not upset - I am picking my 16 for WC.