改天咱们一起吃个饭吧。 i think the above one should be a good answer to your question, it's not literally translated though. but as a native speaker, i think it sounds rather natural and convey what you were trying to say rather well. ”someday“ should be translated as 改天 in my opinion;p You could also say like this :下次找个时间一起吃个饭吧。it's english translation version should be more like:we should go get something to eat SOMETIME. ;p Anyway different languages get into different ways of saying things, you'd better figure that out as you go along, so do i. ;p Hope it helps.

我們下次應該找個時間一起去吃飯

改天聚個餐吧 ! 聚餐 : 聚在一起吃飯聯歡。 如：「下週本班將為四月出生的同學舉行生日聚餐。」

我們有一天就應該一起去吃飯! You can specify when you want to eat (i.e. 明天，下個星期，下個月，etc.）as well as what you want to eat (早餐，中餐，晚餐 或者 中國菜，西班牙菜, etc.)

i agree with luoxiaoye, “改天咱们一起吃个饭吧。”sounds very natural