The nether is pretty plain right now in terms of mobs; only having the ghast and the zombie pigman. I have a couple new ideas for mobs...
1.)Wyrm-These spawn and live in lava. They will attack anything they see, including you. Also, things that look unnatural, like iron blocks, cobble, tnt, ect will also be destroyed. The only real way to kill it is having it take a bite of some tnt though. They will bite you, damaging you for 1 heart/second, and then drag you into the lava. They will attack ghasts too. Their range is 3 blocks from lava, and they could be 7-10 blocks long.
2.)Netheric Wolf-These are praticularly rugged and vicious versions of wolves. They are a dark purpleish-brown, with what looks like lava flowing though cracks on their skin. They can swim through lava, and will attack you on sight. On death, they can drop a soul gem, which if thrown on the real world, causes a tamed one to spawn. They do 3 hearts of damage, and have 15 hearts of life.
3.)Mutated Creeper-These leave fires periodicly where they walk. Upon death, they also can turn into lava. They drop 3-4 gunpowder, and are simply red creepers in appearance.
4.)Planet eater (AKA Your worse nightmare) These giants are colossal Wyrms (about 30 blocks long and 5 wide). Only appearing in caverns that are at least 50 blocks long/wide and 20 blocks tall that contain lava, they can fly around like ghasts. They leave a trail of non-source lava blocks behind them, spilling from their tails. They will also only spawn on "Hard" difficulty. When they make contact with a block native to the Nether, a soil block (Gravel, dirt, sand, ect), wool, wood, or other weak blocks, it destroys them. They send bursts of ghast fireballs at you, 3-5 at a time, and they do ignite the ground. They behave like wyrms in biting strange blocks too, like tnt, and can only be killed by having them eat TNT. They will also eat any other mob they see. Run through a portal while fighting one? It will follow you. Exposing it to water will make it die in the normal world. Drops 3-4 of mixed: Lava source blocks, tnt, gunpowder, fire blocks.