You are a great scenarist.

I am sure you will do what makes all of us happy.

translation please?

1.

Based on this http://tureng.com/search/scenarist :

Sen büyük bir senaryocusun.

2.

I´m not sure but here is my attempt:

Eminim hepimizi mutlu edecek için yapacak.

1. instead of senaryocu, it´s senarist.

2. Eminim hepimizi mutlu edecek olanı yapacaksın.

Would saying "olan şeyi/şeyleri" instead of "olanı" sound (un)natural?

"olanı" will fit better.

we use "the thing" in English a lot more than Turks use şey in good Turkish.

mutlu edecek olan is the (thing) that will please him or her. You don´t need to add şey on the end. although if you do it is not wrong, just doesn´t sound so literary