I've decided that for the Rogue One premier I'm going to make Mon Mothma's costume. Mon Mothma shows up in Return of the Jedi, and her costume is in the Original Trilogy costume book. For Rogue One we saw her already in the first trailer, and it seemed to make people really happy online. At first glance she was easy to recognize because it seemed to be the same costume, but of course there are enough differences in the ROTJ costume and the R1 costume that it will not be possible to combine them, even if 95% of people will think it's the same.
The sleeves are also quite different. (both pictures from RL member jedibarriss)
So yes, the costumes are very similar but with enough differences that I can't combine them. My plan right now is to make the ROTJ costume, since there are still more references for it. I don't think I've seen anything on what the R1 costume looks like from the waist down. Also when I bought the fabric I did it with the ROTJ costume as a reference, and it's too heavy for the R1 costume.
I also find it a bit ironic that when I finally find a costume with the exact hair colour that I'm currently using, then it's a costume with short hair. I still need to get hold of a short wig, since I'm not going to cut my own hair off for a costume.