Sorry for being a smart-aleck in the first response. After I posted it I worried your wife might be handicapped or something, making the sidecar the only option.
If not, I'd do some serious evaluation and experimentation to make sure a sidecar would be less scary to her. If she's ridden in a sidecar and likes that but hates the back seat, that's enough. But if she "thinks" she would be more comfortable in a sidecar without knowing for sure, that is an expensive and risky attempt. My wife rides across the country with me and it's my favorite part of riding. But she's got it in her head she isn't safe without a backrest and won't ride without one. I remind her the KZ1000 we rode when we first got married didn't have a backrest, but you can't argue feelings.
My point is you need to find out what scares her off the backseat (or makes her physically uncomfortable) and see if that will really be different than the sidecar. Or maybe you could approach it slowly with training her on the backseat. I don't mean that in a degrading way, because I'm training myself every day. We just have to work up to things with a process instead of just jumping in.
I know it's not easy. My best riding friends liked what Tammy and I do and got bikes hoping their spouses would ride with them. Then their spouses weren't comfortable beyond going 10 miles to a restaurant once a quarter, so they sold their bikes.
Sidecars are uniquely cool in their own way, so I wish you luck in your investigation.