If Bond 25 is about Bond taking revenge for the loss of his wife then a traditional Bond girl (ie. a love interest) seems inappropriate. The film would still need a strong female lead though and I think the best solution would be to team Bond up with a female double-oh agent. As for casting, I have two slightly different approaches which require different actors.
The first is for the agent to be someone Bond has worked with before, they have history and a sense of camaraderie but without any sexual tension. For this approach I’d cast Lean Heady.
I feel the role calls for a more mature actress and Heady has already proven herself adept at action. She’s become somewhat synonymous with villain roles lately so I’d like to see her join the franchise as a friend and ally.
The second would be someone that was recruited since Bond left the service and her identity as a double-oh would be treated as a revelation. Over the course of the film she’d develop a grudging respect for Bond but their relationship would remain professional. For this approach I’d cast Gemma Chan.
Chan is a beautiful, talented up and coming actor who I think could blend elements of the femme fatale with a highly skilled agent.