I had a look at MPTCP, collecting here pointers to investigate deeper.
Depending on the goal, also other protocols besides MPTCP can be considered:
Considering here the several ways for that scenario: I have a laptop from which I access the internet, and I want to freely move that laptop between ethernet and wifi, without applications noticing. So this solution considers a further system (laptop, raspi etc.) on your LAN, as helper. The ways below are so far mostly theoretical and should be tried out one by one (please beat me in trying them before I find the time ;).
The 'real world' scenario from the article linked above has already a part of this setup, but a) it is just terminating the network connections on the other laptop, in real world one wants to connect to plain TCP servers on the internet. Having the MPTCP endpoint doing NAT could help. b) we still need to make all applications on the mobile laptop MPTCP capable, i.e. with 'mptcpize'. Maybe there is a more elegant way than that.
So ideally, below options get researched, and easy to use howto gets created. From the below approaches, the first one is recommended as it enables MPTCP usage in the most transparent way for applications. The bonding approach is also nice and needs no helper system on the internet.
Collection of some approaches. 'pro' means pro-argument, 'con' is a bad/con argument.
So with this scenario, we have a home with multiple uplinks to the internet, let's say fiber and 5G. MPTCP or something else would then help to combine throughput of both, and to survive one of the uplinks breaking down. For this scenario, we will need one system on the LAN (might be the router) running i.e. MPTCP, and a counterpart on the internet.