So I know variants of this idea have already been floated, but I wanted to detail one specific method that has less room for ambiguity (apologies if it's a repeat!).
Let's say you visit a pub and over the course of however long, you pay for three consecutive rounds. At the moment this would simply display as three separate transactions. It would be nicer (as well as wasting a bit less space on the list) if you could tap a button on the most recent transaction to collapse them together.
In my mind it would not only be more elegant visually, but make it easier to see at a glance that you'd spent £30, rather than £12, £7 and £11. And unlike potentially grouping trips or events, it wouldn't need any guesswork as only two or more consecutive transactions from the same merchant would qualify for this treatment.
As feature requests go I know it's not the most radical, but it's something I've been meaning to suggest for a while, so… there we go!