The 16 3-coach 175s from TfW could be transferred to XC to replace 13 2-coach 170s, giving three extra units to play with. This would make all the Stansted trains guaranteed 3-coaches.
XC to retain their 3-coach 170s.
Send the 13 ex-XC 2-coach 170s to EMT, and together with all 170s (23 units) from WMT, they could replace the whole 153/156 EMT fleet as the numbers of units match exactly.
The 11 Ex-EMT 153s = scrap.
The 15 Ex-EMT 156 to Northern to replace the 150s (the numbers match exactly again).
Northern 150s = scrap.
Very neat indeed.
I realise seats might not match and it's a lot of juggling but would help get rid of some older stock, while giving XC and EMT a fairly decent upgrade.
I imagine the new EMT franchise will get rid of all Sprinter stock from that area anyway but the above means they can get rid of the 158s first and then the newly acquired 170s.
Releasing the 158s from EMT means Northern could then:
Use the ex-EMT 158s (26 units) and the ex-TfW 158s (24 units) to replace the 156s (47 units). This doesn't include the cascaded 156s they'd get from EMT though, so seemingly pointless, but as 158s come off lease, Northen could replace the 156s as and when.
Perhaps Northern should grab the 2-coach 175s (11 units) from TfW and use them to get rid of the 155 class (7 units). Wiki doesn't show the 155s being in use under "future fleet" though.