However, after having a few, errr, "borrowers" return (from abandoned players ofc) with stuffs that I'm already red lining on, I thought - wouldn't it be good if you could prioritise what they bring back.

Cotters already have a priority list - system defined, why not thieves ? And why not let players specify, (although I realise this would require need a fair bit of coding). 

T2 thieves especially - don't you just hate it when they bring back food when your red lining on it - or clay. or stone - and the target is full of gold, gold enough to pay the wages of your mighty army for years, or months, or weeks.... (depends on how mighty your army is)...

What I'm asking for is, ideally, to be able to to specify what thieves - errr "borrow", though that may be too much to ask, although a "drop down list " may be an option (yeah, I know it's not that simple)

Failing that, I ask that T2 thieves prioritise T2 resources, and only bring back basic res if there are no T2 res to be had, and prioritse lowest res type in city they are sent from.

As for what they steal there are ways within the game mechanics to have them steal things other than the basics.