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！
They are currently inactive, but they'll be back in a few years.
・They are currently inactive, but they'll be back in a few years.
「彼らは現在活動していませんが、数年後に帰ってきます（復帰します）」
「活動していない」をシンプルに表す形容詞が inactive（非活動的な、休止中の）です。
・They are on hiatus, but they will return in a few years.
「彼らは活動休止中ですが、数年後に復帰します」
日本 
アメリカ合衆国 
