I can take or leave the nasty 150s (not TfWs ones especially, they're just outdated and grotty in general). But the extra 158s would make a huge difference.
It had also occured to me that a fleet of 175s, upgraded to Northern Connect standard - which I don't think would be a big ask - could be well suited to Chester - Leeds. Anyway, just a thought.
I was thinking more in terms of making the 150s longer (by reforming the 78x 2-car Northern ones into 76x 3-car "hybrid" ones by inserting an ex-TfW vehicle into the middle of 72 of them, and using the remaining 12 Northern vehicles to form the other 4) as I understand that even with a 2019 population, they can get crush loaded on commuter duties and even leave people on the platforms...? And there'll be even more people in 2023, so even though they'd still be 150s, they'd be 150s that'd have a better chance of actually having a high enough carrying capacity! Which of TfW's 150s seemed "grotty" to you...?
The 45x Northern 2-car 158s, if reformed into 3-car "hybrid" ones by inserting an ex-TfW vehicle into the middle, would leave 3 ex-TfW vehicles to form a 46th 3-car "hybrid" 158. The 8x already-3-car Northern 158s added to those would mean 54x 3-car 158s. With more people around than ever before, it'd totally make sense to do this in my opinion. For routes with restricted train lengths, there'd still be the 2-car 156s and 2-car 195s.
As for the 175s, they wouldn't be a bad idea (as long as they didn't catch fire). They were originally First North Western units and they're based at Chester depot, so they'd make sense in the general Northern area.