and what about "מה שאני חושב"? or "את מה שאני חושב "? Are ש or את optional here, or are not used at all in such a case?
This is another example where i left off the אני pronoun and the system said I was wrong. Do i need the word אני if the verb form is single first person future tense and therefore implies אני?
It should be accepted. אסתכל על זה מחר is correct, just not very common, at least in speaking, it's more a written form.
מה שאני יחשוב
No, doesn't work. יחשוב is there male 3rd person future tense: He will think. It can't be paired with the pronoun אני