It's a pretty common mixture. I'm not if I've done green beans with caramelized onions, mushrooms, and almonds, but I've done them with 1 or 2 of them. And I'd suggest roasting fresh green beans with the garlic them mixing with the onions and mushroom mixture and sprinkling with the roasted almonds.
Just don't call it green bean casserole or tell someone you'll bring it and then show up with this. Those who love the casserole will be in revolt.
In my family for about 4 decades, broccoli rice casserole ( called green rice in our family) has been at every Tday meal. One sister decided she would steam fresh broccoli, cook brown rice, and make a low calorie cheese sauce. So a casserole arrived with brown rice topped with fresh broccoli, and drizzled with a cheese sauce. My kids eat these items regularly, but you would have thought Christmas was cancelled when they were presented this healthy option when they were expecting the traditional item. 12 years later the joke is still "Don't let Aunt S make the green rice."