There isn't a different word, per se. You could use "which" if the context indicates that there is a clear choice between a small number of things to look at. That's an entirely different word, though, and it changes the meaning slightly. I am guessing that that's not what you are looking for.
Neuter nouns and pronouns in Indo-European languages never distinguish nominative from accusative. For this reason, "what" has always been both the subject and object form. The non-neuter did, however, distinguish nominative from accusative. Although the accusative has since been lost, the dative took over its function, allowing the language to retain a subject/object opposition, at least in written in English. This opposition is more or less defunct in the spoken language. It is preceded by the loss of such a distinction first in nouns and adjectives, and more recently in the 2nd person pronoun. I imagine in time there will remain no such distinction in any pronoun. Indeed, in mainstream spoken dialects of English, the object form of pronouns is the default, and the subject form is only used when it is the direct and sole subject of the verb, much as in French.
Finally, note that the rule that a sentence should not end with a preposition is and always has been nonsense in English. Outside of formal writing, it's acceptable to end sentences and clauses with prepositions. I wouldn't spend any more time correcting yourself.