Adding new effects for skills.

Duplicating existing mechanics for new skills is currently problematic, but creating many new units with effects from a configuration file (if you're familiar with this, you can take a look – there's a buff that works like a spell) is something Waterfall is actively doing in Armageddon's Blade – you can visit him and see how many new skills he's created.

You can make creatures neutral by removing dependencies on racial perks, but your idea – to make it so that any of the racial perks at the EXTRA_EXPERT level unlocks your creature, is not feasible. In the hero skill tree, there is a logical "and," but not "or"