Yes, there is space for small niche sites with certain angles, but all of those sites will still be owned by one of the big two parents. There isn’t even much space for a niche player intending to eventually get bought out by one of the big groups like you see in some software fields, because hotels are pretty risk-averse in general and won’t just hand out access to their reservation channels to small players easily.
I did some work on hotel booking for one of the the "small" players in the space (at the time, a distant third in the EU after Booking/Expedia). The majority of their room availability was sourced through the two big players. That's probably going to be the case for most aggregators, and there's a fair chance that your favorite non-Booking/Expedia aggregator is actually just selling you their content and collecting around 10%.
Yes, there is space for small niche sites with certain angles, but all of those sites will still be owned by one of the big two parents. There isn’t even much space for a niche player intending to eventually get bought out by one of the big groups like you see in some software fields, because hotels are pretty risk-averse in general and won’t just hand out access to their reservation channels to small players easily.