Sushi using brown rice
Normal brown rice just isn't sticky enough (not enough available starch) . Risotto rice might work if you have some anyway, though the texture will be a little different. Apart from the fact that rice keeps forever, if you've got sushi rice to use up, it makes a perfectly acceptable risotto, or fried rice balls. It doesn't need to go to waste.

Where you're going wrong is thinking that rice is rice. It isn't. There are several major categories of rice which behave differently in cooking. Trying to substitute one variety for another, even though they're the same species, is like trying to substitute sweet peppers and chillies.