I can see how P0 for Piccadilly might seem like a 'quick win', and be relatively cheap, however given that Airport services from North TPE will no longer ned to reverse in the main trainshed is there really a need for yet another terminal platform?
P0, on the assumption that it is to the East of P1, would only really be suitable for services via Ardwick- so only really the Glossop line and some Sheffield services. Anything else would simply replace all the conflicting moves that the Ordsall Chord is supposed to eliminate.
South TPE services (to the Airport) AFAIK will still reverse at Piccadilly, so they would be a candidate for P0 (or P1/2 freed by stoppers moving to 0) but with the North TPE services (4tph altogether during the day) no longer reversing; and presumably post HS2 there won't be demand for 3tph from Manchester to Euston via the classic WCML. I'm not convinced, in the medium-to-long term, that extra terminal platforms at Manchester Piccadilly are needed.