That is supposed to be completed in xxxx. I want to go again when it's finished
That is scheduled to be completed in xxxx. I want to see it then.
That is supposed to be finished in xxxx. I want to go again when it's done.
「完成」はcompleted, finishedあるいはdoneと言います。このケースはどれでも大丈夫です。
「完成する予定」はscheduled to be completedやsupposed to be completedと言います。supposed to beはscheduled to beより少し砕いた言い方です。
「完成したまた行きたい」はI want to go again when it's finished、I want to see it then、I want to see it when it's doneなどと言えます。どれもニュアンスは大して変わらないです。
ご参考になれば幸いです。