I would have thought a Newquay - Plymouth service would be the best thing to do with what is currently available, also stopping at the Plymouth suburban stations as has already been mentioned. One thing I would propose would be more longer distance services on Fridays, Saturdays, Sundays and Bank Holidays though, to/from places like Bristol & Cardiff, and maybe a couple of token ones from/to Paddington and the North, as it would encourage more long distance traffic for weekend breaks (which is what Newquay seems very popular for).
In an ideal world, the branch would be sped up a little, with new track towards St Austell. Maybe 1tp2h Newquay - St. Austell/Truro, And 1tp2h Newquay - Par/Plymouth, giving an overall service from Newquay to the mainline 1tph.
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Not Cornwall but I think the Exmouth line has the same issue as Falmouth. Intensive service with only 1 passing loop.