I don't think anything will carry enough power to even almost 'one shot' another mech, excluding one of the MASTIFF's against a flea-class or something rediculously one-sided of that nature. Even if we were operating colossal 100-ton Tanks, It would be moderately unfair if it could take out a full group of light or flea class mechs in a half dozen shots. Team up six of said tanks with some close-range mechs to act as a barrier to protect them, and you have a line of defense that's near impossible to breach.
However, If tanks are incorperated as playable mediums, I would expect to see a few with very heavy weaponrey and lighter armour, just not of such a drastic scale.
Though now that I think of it, I could see Lucron lining the valley edges with heavy tanks as fire support... Valley battle wagers, beware.
Back on topic, agree. Playing as tanks would be entertaining, and would be nice to see in the game if the Dev's can get around to it. Add a bit of variaty to the meele with little swarms of cannons.