There were some intentional choices here to not put many things high in the tree so players could choose them earlier but still take them at tier 6. We'd rather frontload many of the options. Realistically, some of the tier 5 abilities could be tier 6 (possibly with some number changes, possibly not), but it's better to have the choices earlier. You aren't going to lack for choices when you are reach tier 6 options, it just might be something you've skipped until then.
This isn't to say we'll never add anything to the top tier, but there's no immediate plans.