Homebrew Subclass feature, creature selection

I'm trying to make subclass feature whereby the play gains a low cr beast and plant they can choose each day. At higher level s they would gain high cr beasts and plants ending with them having 4 plants and 4 beasts to choose from each day. 

I'm struggling to have these options show up. How should I go about making this possible?

Make a feature that plays with Find Familiar?

You gain the Find Familiar spell and can cast it 1/LR without expending a spell slot, you can also cast it using any available spell slots yada yada...

When you cast the Find Familiar spell using this ability you can <insert what your after>.

Then just at the higher levels change the <insert info here> to adjust what that spell can now do?

My quick 30 second idea :) GL!