I'd argue that 3+2 seating is fine on 20m MkIII-based stock, though perhaps 2+2 would be better in the central section of each car to allow more standing space in the peaks.
Tables I'd suggest is a no-no unless it's those 'half-tables' as on the 318s/320s- perhaps ones that can fold away like those in the wheelchair areas of 158s & 185s. Reliable and audible PA systems and PIS screens will be required post-2020, so that's a given. In 3+2 areas, some airline seating would be okay in the '2's but I'd avoid it on the 3-seat side due to the difficulty some people would have in getting to the window seat, especially when it's busy! Taking the 333s as an example, all seats on the '3' side are facing bays, whereas the '2' side is a mixture of airline & facing seats.
Returning to Moonshot's hypothetical question, I imagine that if a new unit was to be dedicated to Stalybridge shuttles it probably wouldn't have toilets fitted, as there'd simply be little need for them- Metrolink's M5000s don't have toilets despite being used for fairly long journeys to places like Bury and Altrincham! In addition, any business case for a small dedicated fleet would require costs to be trimmed at every opportunity!
Of course such a move would require a complete overhaul of the way resources are managed on the railways though, hence why the posed question was hypothetical.
With the 319s though, toilets are and will continue to be provided- whether a new, accessible toilet would replace the current one or be an addition to it will be down to the ROSCOs and TOCs to decide.