I know you can already use a contactless card instead of Oyster for travel but hear me out.
If you have a railcard, senior, 16-25, HM Forces etc. You can attach this railcard to your Oyster card and receive the 33% discount on Oyster travel. You can’t get the discount without using an Oyster card. Integration would be nice so I could see and top up my balance in Starling’s App, seeing travel and transaction history too would be cool. If Starling could then set it up so my Starling card was also my discounted Oyster card so I could discard my physical Oyster card that would be a nice touch. This would also, for me anyway, add to the ‘Travel the world’ (including London Underground) aspect of having a Starling Card.