Comment by procaryote

The best advice is probably "don't", as it usually is to most people setting out to design a programming language, and even more so for people setting out to do a mostly backwards compatible extension to a language that isn't suited for what you want it to do.

The second best advice is probably, do just c with classes. Allow defining your own allocator to make objects of those classes. It's fine if objects built with one allocator can only refer to objects built by the same one.

Don't do templates, just do the minimum needed for a container type to know what type it contains, for compile time type checking. If you want to build a function that works on all numbers regardless if they are floats or complex or whatever, don't, or make it work on those classes and interfaces you just invented. A Float is a Number, as is an Integer. Put all that cleverness you'd waste on templates into making the compiler somewhat OK at turning that into machine types.

Very specifically don't make the most prominent use of operator overloading a hack to repurpose the binary left shift operator to mean `write to stream`. People will see that and do the worst things imaginable, and feel good about themselves for being so clever.