It better be Jack.
No Derek, as more screentime for him, makes for a less enjoyable storyline. and I don't want Max and Tanya with more affair misery, not right now.
Also, Kat and Jack worked well, in the babyswap fall out, they have something to bond over, and it would make sense, the whole "love" thing, as Kat has been looking for someone to talk to, someone who listens. And as much as I like Alfie, he is not a good listener, he always gets the wrong end of the stick, puts his foot in and never has the right thing to say.
With the aftermath of the swap, it seemed like Kat and Alfie did not have a clue what the other was thinking or feeling and if you can't udnerstand each other, how can you support each other?
Kat tried talking to Michael, is Tommy, when he was believed to be dead, was his biological son, and she hoped he would understand and Alfie was behaving like this, as it wasn't his kid. Michael didn't get her pain either.
Jack seemed to understand Kat a bit better, probably because they both deal with pain in the same way. That would be a good solid ground for an affair to start from, it would also explain why it is more than just physical attraction.
with any of the other suspects, I don't think love is really what they are after.