Do you think there is a market out there for products that keep your veg and sauces warm at dinner. 
yea, they’re called “dishes”.
Simple answer is yes. Its already done.
http://www.salton.com/product_info.asp?category=Warming%20Appliances&id=1
What specifically did you have in mind?
Use water to keep food warm in the dishes. This would be veg you take when you nee/want it.
like a “Bain-Marie”? they have been used in gastronomy for ages, you can see them at every buffet from Lagos to Tokyo…
but for home-use? I don’t know but I am pretty sure that there is something already out there…
fondue is a similar concept, maybe you can get some inspiration there…
I was thinking about a plate (ceramics?) that could keep the heat for a longer period, that would be cool and every chefs dream (me included)
Put your plates in the oven (lowish temp) before you put food on them. Unless they’re cheap thin plates, they’ll stay warm through dinner. Every good restaurant in the world does this.