They are not doing anything right now but they will be back in a few years.
They are not up to anything these days but I'm sure they'll make a comeback in a few years.
どちらも「彼は今活動はしていないが、数年後に帰ってくる。」というニュアンスです。
1) They are not doing anything right now but they will be back in a few years.
2) They are not up to anything these days but I'm sure they'll make a comeback in a few years.
「活動していない」は
not doing anything や not up to anything を使って言えます。
「数年後に帰ってくる」は
will be back in a few years または will make a comeback in a few years などと言えると思います。
ご参考になれば幸いです！
日本 
