Regardless if it is a solo or not, it's a plot point that has been done. I don't think Cap dying is a logical end point of his character. Dying is easy, living is more challenging. If Cap sacrifices himself, he's not doing or showing us anything about him we don't already know. He is the guy who will do that. For me it's too predictable. I'd rather they be more creative. At least with Stark, there is the poetry that the MCU as we know it started with him, so him dying signals an end to that. Cap's death doesn't have that same poetry.