There is no necromancer class planned at all (not sure where you heard that) and you can use fire magic through the Dragonknight class (just play it as you would a mage). Ice magic(alongside fire and electric) has the destruction staffs as well.
I'm doubting with their current model that they would add another "class". More likely that it would be added in as a "guild" you joined like you currently join the fighters/mages guilds to learn their skill lines.
So you would make a sorcerer and subtype it as a necromancer.
If you don't already visit there, you might head over to the Tamriel Foundry http://tamrielfoundry.com/forums/ . There is a lot more information to be had over there.