How about these changes:
1. Gate does not have a summoning and controlling aspect. It is just for transport. You can still go "I want a gate to a balor", but the balor is not compelled to enter, and you can step through the gate to him.
If you want to get some sudden combat support, then that's what summon monster IX is for. If you want a loyal servant for some task, use the planar ally chain.
2. Shapechange has a duration of 1 round per level. This effectively makes it a solution for a single encounter or obstacle. If you want a long-lasting disguise, there are spells to cover that.
3. Disjunction has a full round casting time. It's still a great, powerful spell, but it's not going to be a "oh, and now I'm losing the fight, so I may as well screw my enemy over".