Obviously the revolution happened and I know "what ifs" are not allowed I am not asking what definitely would have happened just if their was a plan if the Czar-to-be died while young since he had health problems.
If a Czar would die without male heir, the next in line would be Czar's oldest brother or (if he is also dead) his male children. There was a law about that sort of thing enacted in 1797. If there were no male heirs in all the family lines, closest female relative would take the throne. More or less. The actual rules were quite compicated but well defined, without leeway for quarrels.
Alexei Nikolaevich was the only son of Nicholas II. Assuming an early death of the young Tsarevich, the title would have passed, under the Pauline Laws, Nicholas' brother, Michael Alexandrovich, as the closest male relative.
This is in fact what happened when Nicholas abdicated in 1917. Working from the assumption that Alexei was in no position to ascend the throne, Nicholas abdicated in favour of the Grand Duke Michael. This unexpected news put the latter in a bit of a bind (he hadn't been forewarned) and for variety of reasons he declined to accept the crown.
Beyond that, Michael was childless so the next in line was the Grand Duke Kirill Vladimirovich, a grandson of Alexander II. Nicholas' daughters would not have been considered unless no other male candidates were available.