when the zai is placed before a verb,it means the action is taking place.when you put it before places,the zai has the same meaning with the prepositions of time and place.and we usually put the predicate verb after the subject, so zai na li should be placed after the verb xiang.

你想强调的是哪个动作就把[在]放在哪个动词后面，[你在这儿干什么呢]，想表达的是干什么，在+方位词+强调的动作，[你想在哪里吃]，想表达吃这个意思，也是在+方位词+强调的动作，而人称代词后面的[想]也可以换成[说][要]之类的词，比较中立。[你在这里想什么]这句话强调的就是想这个动作了，也是在+方位词+强调的动作，what are you thinking about？

在 works differently in different situations. When it is placed before the verb, it means the action is in progress. Like adding "ing" to the end of a verb in English: i.e.我在看电视- I'm watching t.v. 在 also comes before the location, i.e.: 在这里- [in] here,在那边-over there, 在这边- on this side -just like in: 你在这里做什么？-what are you doing here?(在 is before the location) 想 means "want" (in this sense) and is placed right after the subject. Just like in: 你想吃什么？-what would you like to eat? The same rule still applies. 想 stays after the subject and 在 is placed before the location. So together it would be:你想在哪里吃[饭] (where would you like to eat?) Does this make more sense now? Message me if you need more help :)

你在这儿做什么呢? before the verb 做 你想在哪里吃? before the verb 吃 I think it is self explanatory.

做什么 做修饰的是什么 想在哪里吃，想 着重修饰的是“在哪里” 而不是“吃”