Glad I was able to shine some truth on this character with my earlier deduction xD
Anyway, another stab at the girl theory here with another look over the translation. Beware! It is long and probably pointless, but if you want a chuckle at my slipping saneness, here you go ;3
The title 'A Rival Appears' is all fine and dandy. At first glance, you'd think it'd be for Shinichi. But, reading again I've noticed that the first line is an odd choice and possibly a hint: "Ran, you really are unrivaled!" Probably nothing, but right after that title? Hm, something may be up here. There's also the fact that Ran's yet to give her own confession, leaving only a little room left for a girl rival on her side. Again, probably nothing, but if I were Gosho (and I could be with my punny humor OTL) I'd use this opportunity. Something I didn't notice the first few times either this time is the vertical writing: "Today, a three-person outing" Not sure if it means anything, but that's not very specific.
After this Sonoko then goes on with "He may have a change of heart". Now, I found this weird. As far as I can remember, she's never really been on this track before without it being teasing, but right now she's oddly serious as in 'Ran, you should hurry or you may have competition'. She even says that Shinichi is 'surprisingly well-liked' another thing I found strange. Sonoko here is hinting more at Shinichi being stolen away than she has in the past: as far as I can remember it's usually more like 'Oh, you two love birds, what have you been up to?' 'You should plan something romantic' or 'That geek probably bored you' 'You should aim for a better guy' or even 'Missing your hubby?' '*imitating* Oh! I wish Shinichi were here!' I will say that it's likely things would be different because of the confession, but even when she learned of it, Sonoko called him 'that stinking guy'. She then reacted like usual (IMO) by mentioning it was like Ran forced it and imitating her confession all lovey-dovey. When it came to Ran not responding, she does mention he may have thought Ran 'rejected' him, but it's more of a joke to get her to phone at that moment to do it. Repeating, I'll acknowledge the circumstances are slightly different as more time has passed, but this just seemed weird from Sonoko.
Following her 'well-liked' comment, she says 'One day a powerful rival may appear, you know? <3' Considering the timing of this comment, I really don't think this was meant as 'A powerful rival for Ran's affection could appear' I take that as 'A powerful rival for Shinichi's affection could appear'. And, as mentioned before, who shows up right after this comment? [s]Abs.[/s] Sera. Then there's the first 'pervert' comment. Sera isn't embarrassed, just honestly confused. Then they don't get the time to take in Sonoko's comments as Ran up and attacks him/her.
This is where things get interesting: the fight. Again oddly happening right after Sonoko's 'powerful' comment, Sera proves (for the sake of theory) her strength defending herself from Ran. Not to mention...there's a detail I've noticed in Sera's build. It'll be touched on below, and it's more than likely my eyes playing tricks, but for all good and fun speculation, what's there to lose?
As shown above...that jacket may be more than just stylish. Along with the hat obscuring some details from her head/face and the dark pants hiding erm, male parts from our view, what I've shown here seems very suspect to me. Of course, I'm fully into this theory, so again this could be my sanity slipping xD
Here is where Sera finally gets a little break from the fighting trance with Ran's comment. In the breather, she finally catches up with what's been going on and is now properly embarrassed. As someone mentioned, a girl groping a girl...yeah. There's also more instances of hidden details of Sera's build on this page, but I am lazy
Before Sera can explain, Conan comes in, suspiciously using 'that person' and 'this person' instead of 'he' or 'she'. As for Sonoko's 'hottie' comment, someone mentioned that female-looking guys fall into hot. Not to mention, if Sera is a girl...well, there have already been people on these forums who've said she's cute/hoping it's not a girl, right? That's not the best evidence as anime/manga is different to our eyes than those in the 'verse, but hey, I have to grasp here xD
Now, the 'you're my type' thing. As I mentioned (but no one seemed to notice

) this could've been meant as 'You're my type of opponent'. Notice that Sera never actually said 'girl' either. And think: what if Sera just has a difficult time with words? Not only that, right after she says 'Or something' and laughs. Now, why would a guy with enough gall (and strength not to worry about a punch) why would he say 'or something' and back down. Conan's reaction is also kind of weird. If he noticed Sera was a girl, it makes sense. An argument here could be Conan's next line in all fury, and it would be a very good one. Though at this point this speculation isn't all that serious so yeah. Also at this point is that I notice Gosho seems to be avoiding Sera's chest with all he's got: bottom of the frame, things in the way, dark clothing...I dunno, just seems weird.
On this page, Conan actually says 'he' so bleh, but I'll just go on to say he's come to the wrong conclusion.

Then lastly at the end is another 'this person' at the side. Something to finish this silly idea is the jacket: this is probably just me, as I'm almost certain it is xD, but those types of jackets with two zippers, I swear I've never seen a guy with one. This is far, far from evidence, I just wanted to point it out for some reason xD
One last thing I've deduced: why this chapter was done so fast. It's very simple, really. Abs. experienced this personally, so obviously translating would be easi-*shot*
Aha... Anyone think we'll make 40 pages with the next file?
