I think, speed is one of the main things that make table-tennis interesting. If I imagine two defenders playing against each other - perhaps both of them with spongeless rubbers - this would probably be a very boring game...
One good possibility could really be do forbid speedglueing, I know one player - he does speedglueing - he tries to smash each ball that is a little bit to high... okay - this is effective - but not really a nice match... I think it's much nicer when both use aggressive topspins to make the point...
Another nice playing style - someone who does high defensive balls from far back behind the table (I don't know how this is called in english...
).
The rise of the net height shouldn't be such a problem - 2 or 3 cm are always possible with the nets that are in use at the moment...