I don't actually know any examples. But I once made my own php web framework out of frustration with overcomplicated existing frameworks. It seemed every decision was made without any real thought.
On of those things are ORMs. They are great for really simple relationships but get increasingly obtuse for any real queries. Somethings, like multiple sub queries to the same table, they couldn't handle at all.
I whipped up an RPN ORM for querying XML documents as a proof of concept. I could do any query I could think of with a very simple engine.
Granted it would have been a lot more work to get it to generate proper SQL or to optimize the queries for memory usage etc, but if you wanted to make your own engine you'd be starting with 1/100 the complexity.
I am thinking about it for distributed search. A peer to peer search is always going to kinda suck compared to a central database, but we can make choices to lighten the load on individual nodes.
If queries got passed in a standardized postfix notation, then each node wouldn't have to parse the query before running it. You could still have SQL locally for people that want it, but it would get transpiled locally before the query is sent to the network.