If you make pet abilities that are a replacement for the Ranger's spells... the class is no more complex than it normally is. The only difference is that the game mechanics that would come from of the Ranger when they cast spells will instead originate out of the pet (or pet and master combined.)So pet abilities should be very limited, an attack, movement and maybe a bonus action.
When you start creating different pet abilities you are developing a tiered pet that is a heck of a lot more complex.
If a player doesn't want their pet to do anything more than just move and attack, they don't have to use this new class idea. (Considering the fact that my idea is not actually going to happen in the actual game anyway and is purely a homebrew concept someone would have to make for themselves.)