Mighty and Ray? Julie-su? Uncle Chuck? Dr. Finitevus? (the main series is lacking in echidnas) Ixis Naugus? Mammoth Mogul? ... Snively, even?
Who did you have in mind?
Mighty and Ray are already SegaSonic characters so the only thing is that SEGA needs to bring them back properly.
*ding ding* You got the two right! Julie-Su and Dr. Finitevus!
Since Chronicles is considered non-canon, those are my two choices and I have a third character who came out recently. First, let me explain for the two:
Dr. Finitevus: Its not the reason the main series is lacking echidnas, its that its lacking recurring villains. We need another villain that can bring a proper threat similar to Eggman and Finitevus is a good choice. He's crazy! Not only archenemy for Knuckles but one of the crazy villains that you know Sonic and the others can't turn their backs away when he's around. Plus he has great abilities, including ring teleportation technology, something that has little to no information about it in the games.
Julie-Su: Yes, I know, she's part of Ken Penders' creations... but here me out on this. Out of all of the characters Ken created in the Archie comics, Julie has the most connections/relationships with the SegaSonic characters. And one major thing that the SegaSonic cast lacks is a very tomboyish girl. Amy was about to be this back in the day, but she was retcon to girly-girl. Cream, Tikal and Rouge can't fit into this category. People may say Blaze is one, but I think she isn't because she does have some femine qualities, although she's more reserved. And Marine... well... she just talk alot XD (F*** Wave, even though she is Sakura VA's from both English and Japanese).
As for relationships: she has a rivalry with Rouge, friendship/enemy relationship with Vector (though sometimes it doesn't make any sense), knows the Chaotix, get along well with Sonic, and (this one would've work if Ken left the comics alone and moved on, instead being an ass to start a stupid mess that can mess up the Archie/Sega relationship) a sort-of "big sister" relationship with Amy. And of course, love interest for Knuckles. (Sega can follow something like SNK Playmore's King of Fighters series: As Sonic and Amy is similar to Mai and Andy's relationship, Knuckles and Julie-Su can be like Ryo and King: shy, but they both know they are interested in each other, they just don't want to push it further).
Presentations, design, background, personality and impact notes: I know many of you guys will say Shade is better. Unfortunately, I have cons about her and we are not sure if Sega/Sonic Team will continue Chronicles. I'm going to save the Compare and Contrast of Julie-Su and Shade in a later date. But let me get to Julie..
Julie have the looks of a SegaSonic character, even though many will say "she's a pink Knuckles with hair". Only things she needs are pants and less cybernetics (because many artists are inconsistent with her design, add more cybernetics or less; hair is different, etc). Abilities-wise, she's fine as she is because she's a soldier. Of course, she has a gun but that thing has stuff that can be consider if she was part in the SegaSonic games (imagine if Knuckles has a recent starring role in a game and Julie-Su is playable, I can picture her playing similar to the Lego Star Wars' shooter characters).
Now the only thing is that Julie-Su needs a background change. I'm thinking she's part of echidna group similar to the Dark Legion, except for this tibits: no relatives or family members (as in don't give her strong connections towards OCs that might be relevant in the future), and instead being a basic soldier, give her more abilities that can make her considered to be "deadly" in the battlefield. And if possible, make her at least unique by giving her a last name, since Tails and Amy are the only characters with last names instead of being "The Fox" or "Hedgehog".
And this can go well if they add my third choice: Thrash the Tasmanian Devil.
Thrash is new but he's unique and looks like an exclusive-Archie character created by Sega that needs to be in the next Sonic-spinoff. Plus, his species is endangered, so this background can work well for him. Imagine this:tasmanian devils and possibly other echidnas are near extinct because there were previous dangerous conflicts. So this can help some plotholes on what made Knuckles nearly the last of the echidnas (I really want to say, Last of the Knuckles Clan, but I don't know what's on SEGA's minds).
So those are my picks. Potential characters that can make the series much more interesting (instead of adding more on Shadow, time travels and alternative dimensions).