I am totally for transhuman traveller. As to the question "What makes traveller" to me the one thing you can't change is the travel and communication rules. Traveller has a pretty fast FTL engine, the jump drive but no FTL communications. This makes it necessary to travel to communicate and it means at a minimum it takes 2 weeks to send a message to another system and get a reply. It can take months for information to reach the capitol of a large interstellar polity and the same time for a reaction to reach the border.
This mandates a decentralized government with a lot of variability in different parts of it. You end up with a distant central government that maybe takes a limited role in most matters and focuses on the military as in maintaining a defense force in case of a hostile Alien race being encountered. It might maintain standards in certain things like disease control to keep plagues from spreading to world to world, first contact protocols and other things that could affect the human race as a whole but beyond that leave a lot of things to local custom, authority and so on. So you get wildly varied cultures and societies as only things that can affect humanity as a whole are subject to the central government.
Barring super nanotech and matter replication or synthesis trade becomes vital. Some systems may be better suited to one thing, like industry or agriculture, than others so trade occurs. What may be legal in one system may be illegal in others, so a black market may exist.
As long as it's a local matter it's up to local governments which could be nearly any type imaginable.
That is pretty much what you can't change about traveller and still have traveller. Some worlds may opt to go totally transhumanist, some worlds may ban transhumans. So adding transhumans to traveller is ok with me.