If anything, Tamim needs to play in first class county cricket, and realistically it should be SN and not just Ash playing in the Lancashire leagues/ minor counties. SN would do well to play a bit in the swinging English conditions. Heck I remember in UK tour 2005, he
played a fine MOM innings against Aus in an ODI.
I think both SN and Tamim, along with Sakib have an excellent work ethic that will go well for them to the extent that BD will have our own version of Jayasuriya, Ranatunga, and Aravinda De Silva. (Sakib ≈ De Silva, Tamim ≈ Jayasuriya)
Zunaed, Imrul, Mushi all have too many limitations as batsmen to be considered in this upper tier I know Mushi has solid technique, but he does usually have very chancy/iffy gameplay to set himself up. I wont judge Rokib because I just havent seen him in different wickets and innings enough to form an opinion.
When you have more capable players shouldering responsibility across the batting order, dependence on Sakib alone will no longer be an issue.