I'm guessing Urien or Dudley are probably the best bets for an SF3 character in SF6, Dudley seems to be getting called more but I think you could argue Urien maybe suits the SF6 style more and is a bit more different to anyone else in the roster, especially if Balrog ends up in SF6.
I wish i was optimistic as you are about Dudley, i still feel Rog will just jump in SF6 and close the door* because World Tour pushes really hard on his return, he's already guessing not IF return but wich side will pick (Bison or Ed)
Urien yes i think it's strong candidate even if i think he'll be material for later seasons like 4 or 5
So far Secret Society member who seem closest to SF6 story may be Kolin, iirc she's supposed to be somewhere in Nayshall in current game timeline (even if she's not seen anywhere) but who knows, females in theory will be added 1-2 for year and more popular ones may push her in very late ones or she may be just a NPC like Fang
For what's worth there have been two big character request/popularity polls and in both the most requested SF3ers end up being Makoto followed by Alex, both of times they both ranked n the top5
If i must say 5 from SF3 thinking my taste i would like
Makoto -> decent hopes, she's very popular and skipped SFV
Dudley -> as said i fear he's possibly doomed due Ed/Rog, plus Ken story may involve english gentleman Eagle too
Hugo -> SFV lore support it, but we got nothing else so far. Guess helps Poison it's one of safest bets (i would LOVE Hugo to be like SFV Mika, with Hugo doing main fighting and EX moves getting Poison doing assist stuff lol, would fit them and we get both them with one slot lol)
Urien -> cool char as we said seems just easy to drop him in at some point
Remy -> call me crazy but even if i disliked him, would be curious to see SF6 do its magic and reinvent him into something better
thoughts on others
Elena -> was one of my first request, got it already
Alex -> think he's one of safest bet at some point. In SF3 was one of my fav characters designs, in SFV was probably worst redesign of whole game
Sean -> i care zero about him (give me his sister!) but feel he's another safe bet, we even got art of him training at Luke's place**. Wonder if we will get him at the basketball place lol
Ibuki -> i liked her in SF3 but feel less need of her here. For what's worth her name possibly was on the SF6 wall tags
Yun -> was cool design but too many chinese kung fu already (and i'm still waiting my boy Fei Long), plus Jaime is kinda spiritual successor of the twins. Also with 4 characters for year i don't like half being the twins, maybe if they was a single stance character where you switch between them
Gill -> i want his SF3 story to continue, imho should return be SF Boss problem is SF being infested by Bison's popularity. Guess possibly late seasons considerinng Kolin is already around
Oro -> Not a my fav but devs of SFV Oro were already SF6 devs (Ono was removed mid run), must say they did him good turtle Oro was awesome lol
Q -> never was crazy fan of him... story intrigue me, but guess we'll get G at some point and he may reveal some Q story too
Necro -> never been much fan of him, only cool thing story wise would be him find family by joining Ed's group of misfits
Twelve -> burn it with fire, unless they will use him just as excuse to resurrect Nash
*Rog is last i added to my collection too
**