Whilst it's certainly true that Ward-Prowse would be an upgrade on Delph / Gomes / Bernard / Iwobi, I'd suggest that we shouldn't be looking to find someone better than the dross... (Stay with me, I'm going to explain it).
I think we should be looking to bring in a CM who is better than EVERY other option we currently have, including Allan and Doucouré (both of whom I love). The reason for this is simple: if we only upgrade on the lesser players, then one of Allan or Doucouré remains our "best" midfielder and, as good as they both are, they both have limitations. This means our midfield overall improves only a little bit, and puts pressure on Gbamin to come back and be very good (in order to slowly take over from Allan, who is already 30).
If, however, we bring in someone that makes every fan go "Wow. Yep, that's our number 1 best starting midfielder, no question." then we have a little of the pressure taken away from Allan and Doucouré and the option to rest one of them periodically whilst the other plays alongside the new star. It also allows time for Davies and Gbamin to push each other to develop and establish which of them will be pushing Allan and Doucouré for the 3rd slot in midfield.
It's time to bin Delph and give the #8 shirt to someone worthy of it. Bruno Guimaraes maybe, or Franck Kessié?