Unfortunately, the second goal was successfulness (Whitfield is still battling cancer, and the series itself resumed shooting in the original timeline with a new star who has very big shoes to fill). But in terms of creating a compelling tv show...
Spartacus: Gods of the Arena could easily have been an entertaining diversion. With the ending a seemingly foregone conclusion and with the star of the original show, Gods of the Arena probably should have been a suspense free exersice in enjoying the endless variations of "Jupiter's Cock" that Batiatus is capable of creating.
Instead, Gods of the Arena has taken the excellent plot lines, character development, set dressing, and yes heavy use of the Roman-period to excuse orgies that were Blood and Sands' bread and butter, and has managed to actually elevate the show by adding in a sense of thematic and moralistic brilliance that was just beginning to develop by the end of BaS. It has infused the series with a fascinating debate of "fate versus freewill" that has reinvigorated my belief in the power of genre television to speak to infinite life truths.
On top of that, its taken the violence and sex that had seemed mostly like window dressing (and button pushing) during the original series and laid grief, pain, pleasure, and pathos into them. You feel the death of the gladiators in the arena, the helplessness of the slaves in the ludus, and the desperate lust for power that drives its principal actors, Lucretia and Batiatus, towards increasingly stupid and reckless moves. Knowing where the characters end up gives the series a tragic glow that does nothing to damper the shocks at watching the way that their options were whittled away.
Almost more impressively, the show deeply invests us in new characters despite our knowledge that they weren't around five years later (and therefore are probably dead). This keeps anything from seeming concrete in terms of what we think we know.
Spartacus: Gods of the Arena may be the brainchild of tragedy, but it is a shining example of how great television can remain regardless of the reasons for its creation. Part of me almost wishes they were always planning on telling this story, because it's a great one.