Would it not be better to serve Kings Norton if you were going to serve one of the Cross City stations as if it called at Longbridge it would have to go on to the slow lines.
The cross-city services on the Kings Heath line would be serving Kings Norton, & all the stations between there & Moor St.*
So KN would already have additional capacity being provided.
Serving Longbridge on a semi-fast would increase capacity between Longbridge & Birmingham, relieving some of the pressure on the existing cross-city.
(I suppose you could serve both)
I would envisage Longbridge station being redeveloped, & XC stopping there instead of Bromsgrove, allowing connections with the Redditch trains.
Why would the Ashchurch stop be dropped as it is only on the Cardiff Nottingham's which are stopping at Parkway anyway?
XC didn't/don't want the WParkway stop, as its an additional stop.
If WParkway had platforms for the north/south Shrub Hill trains, & WParkway replaced Ashchurch as the XC stop, then it's more likely that XC would stop all their trains there. (more frequent connections into Worcester itself)
Yes, Ashchurch would lose the current very irregular direct service to Birmingham, but it could have gained more services at WParkway, which would be one stop up the line or ~20min drive away.
[going way offtopic now]
* you can't use any of the existing Redditch or Bromsgrove services for the Kings Heath line, as they're needed for the existing service via University. So the new services require their own stock etc, & if they turn round at KN they won't impact on any other paths.
You might be able to squeeze them in as far as Longbridge.