Hiddleston would be very interesting. He certainly can play the hero who is not necessarily up to all good. So, when it's time for him to enjoy dispatching with someone who was particularly nasty, he can be convincing. The Bond of the Books (TBOTB) -- nod head here, for those who revere and still want it to be THAT WAY and we all know "the Books" means FLEMING -- was capable of being ruthless and cruel.
Hiddleston also seems to fit the bill of being not just tall, but a little thin. TBOTB was just that.
TBOTB's face was not pretty: a little Hoagy Carmichael-ish, with a scar added and a black comma of hair always falling down. Hiddleston is a bit too handsome in that regard, perhaps, but in the movies it works well to have a handsome guy in the part, as long as he is believable in the fights and with being tough and mean when needed.
I'd find Elba very interesting, but he may be getting too old.
Hardy -- too late, and I doubt he'd be interested in more than a couple films, if that many.
I've written before that the real problem is not so much casting -- there's always an up and comer out there at just the right time, if the producers are patient and open-minded. The problem is: What to do now ?
Go back to relatively simple-minded heroic action films ? The Craig Bonds have had gravitas. Additionally, whether they get another film (or more) out of Craig, the next Bond thereafter must be a re-boot. Calm down, it won't be that bad. It will, by then, have been a long time since CR, or, perhaps more to the point, since the prior Bond film and actor before the current one's first film.
Ideas:
Go period with Elba, starting with LALD and have the Service bring the new young guy up to 00 status because he's the right guy for this job ? OK, but would it have been credible --- I am not strong on British history of the era -- to have a man of color -- any color -- serving in the armed forces at officer level (or any ?) for Great Britain at that time ? For all I know, a man of color might have had poor chances getting to be an officer, but may well have been used as a spy and secret agent. Have him brought up from South Africa or somewhere else ? That would drastically change the back-story of Bond but come on, at this point, his back story has been: in the past mostly ignored and, more recently, portrayed very well.
People might say, well, that's just another character now, so why make him Bond at all ? Point taken. But constantly going back with the same history gets harder and harder to do and still be interesting. If the history of GB means a man of color would not be credible, then, since CR was recently played, start over at LALD set in the present ? Too trite if the actor is black because the villain is, too ? No one minds a white hero with a white villain, but OK, but perhaps not an issue if the villain is NOT the same race as the hero. In a remake of LALD, Bond can be black, and the villain can be -- whatever....(Hmm -- bring in Hardy for this, eh ?) The "he fools everyone he's really two people" storyline would not fly, but perhaps two distinct aspects of the man's life could work. Skip MR since DAD incorporated and re-told much of MR. Going back a bit, Hiddleston could very well play an English Bond set in the 1950s, though, of course, after making a few period films with him Elba thereafter would more certainly be too old. Either way you start this idea out, it seems you get either Elba or Hiddleston, but not both, unless the first actor plays just a couple films, perhaps produced quickly in the manner of the LOTR and Harry Potter split-books.
If you go period with it, though -- with whichever actor -- there's a particularly helpful aspect to it: It helps to wash the board clean for starting over after the Craig films. This could be THE most interesting and helpful aspect of it, production and story-wise. Then, after a few period films, it helps in starting over again with a (then) present-day Bond again, and a new actor. With some of these actors you're getting them for only a few films (or two...or one), which stands in favor of them doing a couple or three period Bonds. Plus, the period technique may well be spent after three films or so anyway. Another neat aspect of a period film could be using the post-Fleming writer's work. Some of it is distinctly set in its own day and that day now would be "period" (Colonel Sun), and some of it deliberately in Bond's "classic" 1950s days (Trigger Mortis).
If you go period with Elba in period -- if he's not too old if there were no historical feasibility issues -- then, after that, you'd be looking for someone new. Hiddleston would be too old by then, Hardy likewise, assuming a passing of 9 to 10 years. Again, as stated above, it seems you get one or the other unless you pursue the pair-of-quickly-produced period films before going modern again.
There are other things which could be done to "wipe the board clean" and set up the next Bond thereafter, too, such as making the Young Bonds in period before staring up a grown Bond series. I'd have no problem at all with Young Bonds being release in staggered fashion with grown Bonds, though they could not be strictly coordinated since a Young Bond in period becomes mature at a period still much earlier than the present. Even if your Young Bond were white and the grown Bond a different color, why not ? It all won't fit anyway in terms of timelines. Hmm...make the Young Bonds AND the Grown Bonds in period, with Hiddleston as the grown Bond, and have things tie in between the two staggered series. Could be quite clever.
Well, enough rambling ? The bottom line is: there are a variety of interesting and entertaining possibilities, and I'll say this: Elba already looks good setting speed records in modern cars of this day, and both Elba and Hiddleston would look terrific behind the windscreen of an early battleship grey Bentley with a blower and a hidden long-barreled pistol ! Further, if you think it's tough casting a Grown Bond, how about trying to cast all the players for Young Bond, when you REALLY want them all to stay with it, not become little monsters, and carry it through ? In that regard, what the Potter producers accomplished was just amazing.