IIRC Blackpool North - Manchester Airport / Liverpool Lime Street services are some of, if not the busiest services on the Northern network so even if they would just run on those routes that'd already be a good use for them.
Plus, there's always demand for more mainline services. Depending on capacity (which there probably isn't), an additional tph from Liverpool or Manchester towards Lancaster, potentially via Golborne, would be well used. So would another Newcastle - Darlington (probably no capacity). In fact there may be a case for having some 350s located in the North East for ECML locals: Morpeth, Chathill, Newcastle-Darlington, and I would say the TPE Edinburgh too, but that seems to require a 125mph unit.
Rather than direct replacement of 323s, there's some merit in replacing 331s instead, allowing the 331s to be cascaded to replace 323s. 331s can and do occasionally run on the Glossop line.