まだほかにもある what is the subject in this sentence?

I understand this sentence means "there should be other things." But I just can't wrap my head into its structure. If I parse the sentence like:

1. ほか　＋　にも, I can't explain the に here.

2. ほかに　＋　も, then the sentence is missing the subject because ほかに (in addition) is adverb.

Please show me how to parse the structure of this sentence. Thank you!