You can upgrade assasins to night watch to shadow riders. I think they do not lose marksman but I could be wrong.
So your aeron chosen can be a shadow rider.
My favorite Esus civ is the lizardman civilization from Orbis. I think it is called the Mazatal and may be available in other modmods.
These lizardmen play very differently if evil or good. If evil, the civ is very Esus focused. They get national units and a hero that are disciple units with marksman. These units are incredibly powerful in part because they are disciple units meaning they can receive the command promotions (up to command III, I think). This makes converting channeling units, like archmages, a breeze because those units are targeted first by marksmen. Plus, the units must be upgraded from a unique priest unit or assassins. Upgrading from assassins allows for powerful disciple units with flanking and mobility II. Impressive.
On top of that, there's a loophole with the civ. They have an evil hero, outlined above, and a good hero. The good hero will leave the civilization if it is no longer good, but the evil hero has no such limitation, although the civ needs to be evil to build the evil hero to start. This means that you could build the evil hero, convert to Order, and then build the good hero. In so doing you'd lose some advantages, but having two cultural heroes plus the national disciple-marksman units is pretty sick. Of course, it is a loophole so it isn't exactly fair.