Not very hard. They just need to inherit sizable portions of territory, while keeping the capital in Liechtenstein.
That would be incredible hard, until 1719 the Liechtenstein family was a knightly family (even if the had the title of Prince already before being raised to a princely family) with vast possesions in the Habsburg domains and served the Habsburgs, they bought up the barony of Schellenberg (1699)and county of Vaduz (1712) which was united and raised to a principality in 1719. As a knightly family they wasn't potential marriage partner for the greater houses, like the Habsburg, Hohenzollern, Wittelsbachs, Wettins, etc.
In themself these new domain was borderline worthless, and nowhere near as good as their other possesions, what made them valueble was that it raised them the family to a princely family and gave them a vote in the diet. They also had another purpose the two small lordship raised the Liechtenstein to imperial knight (answering directly to the emperor rather than to a prince*). The imperial knights was often given positions as Prince-Bishops. The Liechtenstein could hit the jackpot if they posses a big Bishopric in one of the secularisation waves, with a little intelligent politic they could end up Duke of medium size state, the problem are that such a state are unlikely to take the name Liechtenstein, through it may be possible if several states merges.
A alternative are the fact that the Liechtenstein was also (knightly) dukes of Troppau und Jägerndorf in Silesia, which could end up a independent Duchy if the 30YW turn out a little different, maybe they unite two duchies as the Duchy of Liechtenstein. From there they are in a good position to end up expanding their possesion in the neighbourhood, with a lot of luck they could end up with most of Silesia, which while no major power, could be a easten Belgium (rich, prosperous and industrial).
*even if that prince also happen to be emperor