My two main mid-screen BnBs off a low and overhead starter:
1 Meter Low Starter - F4, 1, D2~Hell Sparks (MB 3rd Hell Spark), Dash, Jump Up Punch, F221+3~(Special of Choice/Leave Neutral)
No Meter Overhead Starter - B3, Run~F221+3~(Special of Choice/Leave Neutral)
For the 1 meter low starter combo, it used to be NJP instead of Jump Up Punch, but ever since the F221+3 string damage adjustment, NJP would just increase the chance of dropping a combo that provides the same amount of damage.
If you cancel into Dark Beam at the end of F221+3, it is to go in for more pressure, bait an armored special, go for an overhead, or go in for a grab.
If you cancel into Hell Sparks at the end of F221+3, it is to set-up some zoning
If you cancel into Sceptor Slam at the end of F221+3, it is to place your opponent in the corner
If you choose to remain neutral at the end of F221+3, it is because you believe your opponent will not expect it, or your opponent does not have anything that can beat your F4, or your opponent is going to expect you to not expect them to expect you that you are going to remain in the neutral and they are going to poke once in the neutral and not be able to react when you block their poke and then strike back with F4, 1, D2...or you can go into a grab, or even go in for an overhead which will be easier to combo off of into F221+3 because of the short distance you landed the overhead.
Keep them in check with your unbreakable (DB+1).
4 is +2 on block and it pushes them the right distance in order for all 3 Hell Sparks to connect on block (like D4).
MB'd Hell Sparks on block:
1st Hell Spark MB'd - You are going to go in for pressure, a mix-up, or to bait an armored special
2nd Hell Spark MB'd - You are giving making yourself safe, providing yourself space from the opponent, and you are going to probably try to zone (HS/Dark Beam), use the unbreakable, do a run in jump in 2 (more range than Jump in 1), or you are feeling risky and you decide to go for Sceptor Slam (non-backdash-able I believe).
3rd Hell Spark MB'd - You don't want opponents such as Cassie, Kotal, or D'Vorah to run in on you, you know that your opponent does not know that they can backdash it, you know your opponent backdashes it but lately you haven't been meter burning it and you believe they are going to try to run in on you if you don't MB it, or you want to be extra safe and guarantee some zoning opportunities.
That's all I can think of for now; have fun