Trying to be logical about what routes would be best priorities (and not just suggesting all of my local routes...), I guess you are looking at the following criteria?
a. routes that spend a large proportion of their distance/duration on electrified lines
b. routes that could share depots with"pure" 319s (to keep things operational efficient, rather than having a micro fleet)
c. routes where air pollution is an issue (e.g. at busy city centre stations)
d. routes where the current DMU could run faster on the electrified sections, therefore freeing up capacity/ making things more reliable - e.g. a 75mph DMU might be problem on a fast main line, but a 100mph DMU could be a lot better)
Uckfield would be great but AIUI the 20m bodies aren't suitable for a tunnel on the line, so I've discounted that route.
Manchester to Windermere makes a lot of sense, given the large amount of the distance that is under the wires - the only question for me would be whether the Windermere branch requires regular services to Manchester, given the opportunity cost of paths on the busy bits of lines in between (Windermere is the kind of route that tends to get a lot of focus/ love on here, because people have soft spots for scenic branches, but using a path through Castlefield/ Bolton/ Preston etc comes at the cost of not using it for other services).
The Manchester Piccadilly - Buxton service would certainly tick all of the "boxes" I've listed above (electrified as far as Hazel Grove, 319s already operate around Manchester, air quality at Piccadilly isn't as bad as at some city centre stations but could be better and being able to path them at 100mph towards Stockport might be a better use of capacity_)
There are various other services around Manchester but I'm loathe to suggest many of them as they are really routes that shouldn't be operating in their current guise as they are too complicated (e.g. Southport to Alderley Edge really ought to be chopped in two).
A local route to me would be the Dearne Valley stopper - there are obvious issues with a slow DMU on the line from Leeds to the junction at Moorthorpe - plus it'd mean that the capacity was equivalent to the other stopper on the electrified section (the Doncaster EMUs).
The Fife Circle is a left field suggestion - there are no other 319s north of the border but, given that there seems little scope of electrification over the Forth Bridge, it'd be a way of improving pollution rates at Waverley/ Haymarket, the 319 would be suitable for that kind of "commuter" route, I don't know if the platforms could take 160m long trains though, or are optimised for (6x23 =) 138m instead?
Some suggestions seem a bit... optimistic... e.g. having a micro fleet for the three units required to run the S&C... but there's some good suggestions too.
