I think this is a regional thing in many parts of the country. Also, I think the thought behind this is that the person using the word "Ideal" wants to emphasize that they have the best idea or that they want others to come up with high quality ideas. I think it would be better to just say, "Come up with an idea and make it your best." But, maybe these people have seen people come up with lazy, stupid or meaningless ideas and they want to emphasize that they want quality ideas. It is incorrect usage, but I think I understand the motivation. Also, once this becomes a regional dialect, it is hard to change. If someone wants to change, they will. If they are not motivated, they won't. Since English keeps changing, it will be interesting to see if the two words don't somehow merge in the future. As for me personally, it grates on my nerves when I hear people say ideal, when they mean idea. But, it is more important to be gracious, so I hold my tongue and say nothing.