In case of incorporating extra-imperial lands and losing the imperial crown, the Habsburgs would have de jure overlords. In case of not incorporating extra-imperial lands and losing the imperial crowns, the Habsburgs would at least be independent there.
The Hungarian nobility wouldn't allow it, as it would increase their dependence.
It would've been required to get approval from the Diet, which was less than keen to allow the Habsburgs to solidify their power even further.