I'm guessing that steven p was hinting that the slopes will flatten after a while.
I don't think there is much of a concern with anaerobic pockets unless you are sticking something organic underneath there. And even so, if you have plants in the tank, they will probably end up putting roots all through the substrate and lessening the danger (Walstad has a section in her book about it).
I've seen people use inert substrate (gravel, etc.) stuffed into nylons to form pillows/sandbags to take up dead volume (don't waste the expensive stuff), and build contours, and then add the nice substrate on top.
I've never done much with slopes, but if you can put in some sort of barrier like Jonny said, it should help prevent the leveling out. Some substrates will probably be better at holding a slope - anything with rough/angular grains will be more resistant to leveling, but I don't have enough experience with slopes to give any specific recommendations.