Sven will *not* be in rehab, at least not for substance abuse.
It's been established by Dora that Sven is the sort of person who has faced no real adversity in life - that's part of why she resents him so much. So even though we've seen him drunk/talking about being drunk on occasion, he's almost certainly the kind of dude who can dabble casually in things and be none for the worse. Or perhaps the kind of guy who used to be way more into taking substances, but managed to lower his intake and get his life together without actually getting "sober" per se.
The most obvious secondary character to have in rehab would be Jimbo. It would actually be pretty funny, because Jimbo embarking on a major life change every time he pops back into the strip is a pretty well established convention. That said, Jimbo is almost certainly older than the main cast (e.g., over 30) so I doubt he would be in the support group.
Steve is the only other real possibility. We know he's gone on amazing drunken benders in the past. However, we also know that within the last few months of comic time he was sharing beers with Martin and Sven, so he's clearly still drinking casually.
It's possible that the reason the rehab ad seemed so tailored to Faye wasn't a Chekov's Gun, but to make explicit how much Faye does not want to go through formal rehab - to the point that even one which makes clear it doesn't involve religion or creeps still turns her off. Her evasive answer to Hanners is a bit hard to read - still it seems more like bravado than anything - something she's turning around in her head, but not willing to share.
I think if Jeph goes the rehab route, it's going to be in part because it will allow him to show new characters as well. We may see some older favorites, but I'm 100% sure we'll see 2-3 more guest characters weave their way into the story as well. Meaning the pace will slow down even further.
