I think the best way to make Starstill more aggressive is to add the following keyword in :

Ethereal (This creature can't be blocked except by artifact creatures and enchantment creatures.)

This helps solve our lack of evasion and enforces the set's themes quite clearly.

Also, I think we should give something to help its creatures kick ass, too. I think the two best options are Annihilator and Provoke, though I'm open to other suggestions. I would go with Annihilator, because a) giving it to big ole' beasts and demons is quite a different take than giving it to the Eldrazi, b) it interacts interestingly with Mastery, and c) it's a very visceral effect that would give Maraka more identity.

Lastly, Steinhauser's Blitz mechanic could also help. Or maybe Unearth? Put it on artifacts too for an added twist? That could be cool.

Thoughts? The floor is open!

Sorry I put off responding. I was trying to tie up all my loose ends before committing myself to this and I am terrible at loose-end tying.
I am 100% for Ethereal. That is a genius mechanic and it fits our set like a glove.

I love provoke in the abstract, but I'm not sure if it does anything special for us.
Annihilator is a better fit. Like you said, it produces some interesting tension with mastery. It doesn't seem like a problem adopting this from the Eldrazi (this is a pretty wrecked world after all) but if WOTC makes another Eldrazi with annihilator then the mechanic will be firmly entrenched in people's minds as Eldrazi-only. Also, most of what annihaltor does can be done with a couple sacrifice effects.
I like Blitz best. Helps aggro and it has good graveyard interaction (which we could use more of). I would reword it slightly to be workable on all permanent types. That way you could use enchantments/artifacts as pseudo combat tricks. If we don't reword it, then maybe split it into two mechanics: one for creatures (with added haste) and one for noncreature permanents (with added flash for combat trickery).

Ethereal seems like a bad Intimidate to me, but there is room for that in this set. It needs something and Ethereal fits the bill mechanically and flavorfully.
Annihilator scares me and I'm worried the design space is limited. It suffers from the same thing as Indestructible in that it is too good which actually shoots design in the foot. You'll have to make it have crazy costs, like:

Annihilator Red Guy
Creature ─ Elemental
: This gains Annihilator 1 until end of turn.
3/2

or

Annihilator Red Guy
Creature ─ Elemental
If you control an artifact this has Annihilator 1.
1/1

Or keep with the "if you cast an instant or sorcery" theme to grant annihilator.

Hmm...I'm intrigued now on how to make Annihilator (damn I hate typing that word) work at all rarities and mana costs.

Blitz is fun, plus its tried and true. It plays nice with oblation since my guys going to die anyways might as well trigger oblation.

We don't need new mechanics to encourage archetypes. Keep in mind that Starstill already has three or four non-evergreen mechanics that players need to learn. We just need to make cards with Constructed archetypes in mind, and print strong cards in all colors that push a variety of decks.

RtR had five. I don't think we've gone over. Ethereal at least should be squeezed in. (And I think Landcycling should be in there too, but that's just you and me who think that AFAIK. Maybe a cycle or two of basic land cycles would sate my obsession.)

I'm ambivalent on Ethereal.
I (still) like Blitz. I might want to see it over Oblation.
I don't want cycling, but if we use it, it should be to have typecycling, not landcycling.

Engrave
Relic
Mastery
Oblation
I think that's it.
Seems like we don't have a returning mechanic.

I think the best way to make the set more aggressive is to put more aggressively costed creatures in it.

Bring back the evasive 2/1 for Kappa

Starstill will have a great constructed impact.

I'd just pick some creatures that already have aggressive stats/abilities and drop their costs by (or more) to make them more pushed/competitive.

I feel like we should cut oblation and replace it with [card type] offering. Clearer flavor, more design space, and allows for some fun, cheatable bombs.

Offering isn't really a good mechanic for commons though.

As much as I dislike oblation and feel card types gives offering new life, Cato's got a point. In Kamigawa offeing only appeared on one cycle. I wouldn't mind it in the set but we can't use it in huge amounts. Not that we have to limit ourselves to a five card cycle at rare. One or two optional sacrifice effects never hurt anybody.

OK, how about replacing Oblation with:

Homage: As you cast this spell, you may discard a card that shares a card type with it. If you do, this spell's mana cost is reduced by that card's mana cost.

(We can still keep some cards with Oblation, they just won't be ability worded.)

The last thing NGA constructed needs is more free spells.

Offering > Homage.

Nah it has tons of unplayable free spells.