This is a tough one.
I really liked Speedle, and like others said, one of the great things about his character was that he had a sense of purpose with his job, but not a preoccupation with justice or a hero complex. He could separate himself from the situation and look at things objectively. It's what made him an excellent CSI. Plus, he was a very enjoyable character overall, with an odd sense of humor and great chemistry with the entire cast.
However, I can't imagine bringing him back in any kind of non-supernatural scenario, which doesn't fit with the series. I'm sure they could go with the obvious "faked death to do undercover work" explanation, but it would seem a little out of place to bring him back to Miami after so many years. Not to mention that I can't see how they would play off the realistic-looking death scene, other than if they directly copied Horatio's plan from "Resurrection" that involved pre-drawn blood placed over a bulletproof vest...except didn't he end up with blood in his mouth? Would they explain that away with "ketchup!"?
I especially couldn't imagine Speedle returning to CSI with Ryan there. Not that I think Ryan is nothing more than just a replacement for Speed, because Ryan is without a doubt his own person with a different set of strengths, weaknesses, quirks, and style of investigation. It's just that I can't picture how he and Speedle would get along. My instinct tells me that Speedle could never get past finding Ryan "annoying", and that would screw up the chemistry of the characters as a whole, which wouldn't make the show very pleasant to watch.
I liked Yelina, but after the more-or-less "closure" of the Raymond Caine storyline, plus her move from government employment to private detective work, I feel like she wouldn't fit in as a full-time character. I wouldn't mind her as a recurring one who appeared in a few episodes every season, though. And how about Ray Jr.? How's he doing? They should do some sort of followup on him, even if it's just in one episode.
I never particularly cared for Dr. Price or Lt. Megan Donner, so I would prefer them not to be brought back. I was neutral on Jesse, so I have no real opinion over whether or not I would want him back. But again, there is a logistics issue with that as there is with Speedle. And the only explanation I think they could give is that he faked his death either to do undercover work or to go into witness protection.
Ultimately, I voted for Alexx. I adored her. She was a wonderful character who combined knowledge with passion, but was able to remain objective at the same time. She was human, and had a great mother/children dynamic with the rest of the team. I would love to see her return. I also enjoy Dr. Loman, and I wouldn't want him to leave. I could actually very easily see both of them being M.E.'s on the show, and getting along quite well. I imagine they might balance each other out- while Tom definitely feels sympathy for victims, what he's most passionate about is the science of forensic pathology, while Alexx always seems to be very knowledgeable and interested in the science, but slightly more passionate about honoring the victims and doing her part to help the team achieve justice. I'm definitely able to see Alexx bringing out a more openly sympathetic Tom, and Tom helping to let us see Alexx's humorous side (which I already love) more often.