I may seem to be in the grip of some sort of late-Winter/early-Spring frenzy of procrastination frenzy but I am totally working on really cool things! With Paul! Also, I'm procrastinating a lot. But with a girl! Which isn't as bad, I think?
Mr Terrific - Would there even be a Mr Terrific in the Thirtieth Century? Terry Sloane started crime-fighting because he was the very best at everything and was super-duper suicidally bored by it all. But was he smarter than Brainiac 5? A better fighter than Karate Kid? More rolly-polly than Bouncing Boy? No, no and no. I figure that a Mr Terrific somehow thrust into a legion audition would find himself feeling suicidally inferior. He'd probably end up living in the sewers underneath the Clubhouse, eating future-vermin (astro-snakes! cosmic sand fleas! plasma eels!) and writing bad poetry about rejection. And everyone would forget about him until Five Years Later, when he shows up in two panels before getting shot by the Dominion. Poor guy.
Sandman - In contrast, Golden Age Wesley Dodds is just too damn hard to stop for him *not* to get into the Legion. Seriously, I don't know about his solo adventures but in the old All-Star Comics days he was easily the most bad-assed of the normal JSAers. As far as I can figure it, Lightning Lad or someone would have a scary dream about Sandman (and possibly also Sandy, the Golden Boy) punching him in the head and then he'd wake up and go to get a cup of coffee to steady his nerves and then Sandman would bust through the door and punch him in the head. He wouldn't even have a vote - no one would. they'd all wake up one day and Sandman would be just finishing screwing his name-plate onto that big horseshoe-shaped mass podium they have and that would be that.
Spectre - Well, he's got the power, but I don't know if his methods are in line with standard Legion non-homicidal practice. Also - and I know I made a very similar joke about Dr Fate - I think that "as old as time" is a bit too far past the age of eighteen to be ignored.
Starman - Let's ignore the fact that he wouldn't get in in a million years because he gains his power from a Cosmic Rod (and that thing really never did work too well, to tell the truth. It seemed like he lost it or found that it was completely useless about every other adventure) and think about how much more interestingly confusing the Legion would be if he got in. Star Boy! Starman! Together at last! Oh no, here comes Starfinger and Pulsar Stargrave! Aiee!
Wildcat - If his "nine lives" power was in effect, Wildcat would totally get in, but even if it wasn't I think that he would be a valuable addition to the Legion lineup for one simple reason: costume diversity. Essentially, most of the Legion (and a decent percentage of the JSA, actually) are wearing spaceman clothes with some sort of fancy-pants logo on the front. Whither the themed costumes of yore, super-teens? Wildcat, with his right-down-to-the-jowls attention to detail (okay, I haven't actually seen a jowled cat, but there must be some reason for those little flaps) could show those young punks a thing or two about looking good while cracking skulls. Picture brainiac 5 wearing a hat shaped like a brain. Are you delighted? The answer is yes.
Wonder Woman - The Justice Society's secretary, which is kind of heinous, yes. It's not quite as bad in the context of the stories ("We can only have eight members for some dumb reason, but we think that you're cool! Want to be our secretary?") but yeah: stupid. Despite (because of?) all of the bondage, Golden Age Wonder Woman actually kicked a fair amount of ass, and her own comic was delightfully weird. I can't really think of any powers that she has that aren't duplicated by half of the legion membership, though. Maybe having Etta Candy around could count (Etta Candy is the most delightful Golden Age sidekick since Slam Bradley's pal Shorty. So I have decreed). If not, I'm sure that she could get into the Wanderers, no problem.