As I've read,
Next reply sequel will be 1974, so far this is the speculation. However there are some rumours that it could be 2002 (it seems that's what the fans prefer).

Source: 
Korean



Which one do you foreign fans prefer? I'm not a follower just curious.
If the options are 1974 and 2002, I would rather 2002... But will they make another Reply? 
2002 was when Korea won the World Cup, wasn't it? That would be interesting. I also think the '70's would be cool too.
iam worried the 70's maybe too sexist for my taste. since this drama is know for realism. so 2002 plzzz
I'd love 2002. All I really know about 74-75 is the political atmosphere and I don't really know how well even that would lend to a series like this. 2002 allows for a much different demographic too, which would be nice.
Oh I would love 2002 in my opinion. I think 1974 is too far back for me lol. I still remember being a kid in 2002 at least.
I'll go for 2002. That's totally my high school era!!!! Hahaha... >_<
CherryBunny : South Korea never won the World Cup. But in 2002, it was the host country, with Japan, and its team arrived in semi-finale. :) It was a big time !

I would rather watch 2002 than 1974 because I could compare with my own life, I suppose. But 1974 would be very interesting and I'm sure I would learn many things I don't know yet.
Oh I thought they won it. Haha! I just remember everyone was going crazy then and going Go Reds! I used to play dress up games online back then and I totally remember the the Candybar dolls dress up games had all the Team Korea Reds outfits and I was like what is all of this? :p Btw, if you don't know what Candybar doll is I have one on my profile. (*^_^*)
I would love to see 2002! I'm such a huge football fan and it's really amazing how the fan culture and excitement of the time of the world cup is portrayed in Kdrama. 
I'd be happy with them going either way as I just really enjoy this series. It'd be nice if they could tie whichever one into one of the other ones somehow, sort of like the 1997 crew popped up in the 1994 series.
1974

2002 won't feel like 'Reply'.
I was hoping Reply 1981 or 1982...so much. The next best would be late 70s. 2002 is WAY too recent.
I would love to see 2002 but I'm afraid they might extend the timeline to 2010 which won't feel reply-like at all.
What nostalgia is there in 2002. Aish! That's partly what made 1988 so special!! Those who are younger, you will be privy of a glimpse into a time you never got to see. Enjoy the experience. In 10 years they can do 2002.