How warm is bag for you? With a tent and baselayers and a hat, is it comfy for you at 25F? Or only down to 30F? Or even 20F?
The point i am making is, how much colder will it be than the lowest temp you are comfortable at without those measures?
After that, I would first use high loft clothing, if it fits inside, since it's gear you will want any way, and it's nice not to have to get up to pee or in the morning and have to get dressed first.
My second one would be an wide overquilt, because you could use it for other things in the summer.