I think this is what most people get hung up on when it comes to planning menus, especially on a plant based diet when the main course is already plants ;-). And personally I love a nice one dish meal, something with grains, beans, and veggies all thrown together.
Even a less than ideal one dish meal beats fast food. But, sometimes it works out to put just a bit more effort into it and serve a few extra dishes along with the main course.
Salad and Rolls are favorite and often seen accompaniments
Simple on veggie side dishes, steamed or sauteed are also nice; broccoli, edamame, carrots, kale, sweet potato, etc.
Baked potatoes work as a side dish, when they’re not part of the main course. 😉
Roasted veggies, like the broccoli from Post Punk Kitchen are also an option
Basically, anything simple that rounds out the meal, contrasting colors are nice; if the main course is green use sweet potatoes or carrots, or if it is orange use spinach or beans as a side.