Whoever it is they will probably be looking for more than 5 films in 16 years so someone younger who isn't going to be delaying and putting things off because of the physical demands/injuries like Craig was.
*Also they've been pursuing a "one last job", "coming out of retirement", "I'm too old for this shit" plot line for the past 3 films which is going to get pretty stale soon so a young up and coming 007 is needed to refresh things so you can't cast a 40+ year old.
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@Del_Bosque's
shout of Dev Patel would be an incredible casting, maybe he just needs a bit of persuading like Craig did - IIRC it took the Broccoli's 2 years to wear him down.
Whoever it is, I hope they learn from the successes of the last 3 films and the Marvel Cinematic universe and realise how much better the films can be if they have storylines that span multiple films. I thought No time to Die was excellent because Spectre established many of the characters. You still want them to appeal to newcomers, but I'd love to see them sign writers or directors to multi-film deals as well as the new Bond.