I'm thinking seriously about a sidecar replacement for my Spyder. My use would be several trips to rallies each year--most likely BMW rallies since that's what I'm familiar with, and they match my style.
Having had a long-term relationship with BMW machines and owners for many years, my first thought is a BMW R1200R. Contemporary features, high torque,shaft drive, and a front end that can economically be modified for reduced trail. Needs added subframes to attach sidecar. Oilhead final drives have been problematical, but sidecar use doesn't seem to exacerbate. New engine management/CanBus electrics increase maintenance issues. Overhead cam heads have shims rather than screws/locknuts. BMW Reputation for overall reliability, lots of farkles from Touratech, and others. Special machines can be ordered with customer choice of options such as non-ABS and wire wheels. Few dealers, but most are enthusiast oriented.
Suzuki Bandit 1200: gobs of power, a full perimeter frame, and simple electrics. No plastic to cut through. Telescopic forks would need to be replaced with a leading link. Chain drive allows easy final drive ratio change, but also lubrication, and replacement every 25,000 miles or so. Build quality and paint much less than BMW. Engine maintenance (transverse four) would be more effort than BMW. Significantly lower purchase cost than BMW. More dealers, but dealers not necessarily enthusiast oriented. Few owner forums.
Moto Guzzi something? Guzzi have some new models with increased HP, but comparatively easy valve adjustment. Telescopic fork would need replacing with leading link. Complex final drive suspension reliability is unknown. In the past, Moto Guzzi service by owners has been more straighforward than BMW. Much more rare infrastructure, such as owners' forums, online parts, etc. Few dealers. and not all are enthusiast oriented. Future of brand always in question.
Harley-Davidson V-Rod. Lots of power, large dealer infrastructure. Would require leading link front end. Liquid cooling would require more maintenance, but adds to engine reliability. Ease of engine maintenance by owner unknown. V-Rod is more-or-less separate from the H-D fanaticism, which for some would be an advantage.