You may want to check this thread out for several budget cards. While these cards are all fairly inexpensive most people who frequent these boards would vouch for a good deal of them being staple cards or cards that have been in their cubes at one point. Also while you say White and red support aggro it looks like you are rather lacking in one drops in both colors for that many cubes supporting aggro tend to go a bit deep on them. Green feels very barren on 4 drop creatures which I'd recommend grabbing Surrak, The Huntcaller and Deadbridge Goliath to add a bit more beef to that slot instead of just having one 4 drop in green. Also are cards like Unburial Rites counted as a Orzhov card? If so you can change that under edit list finding the card and changing the color profile on the card. Also you may wish to balance out your multicolor section to even number of cards per guild and add more artifacts.
A quick look at the cube I don't see a single wrath spell in white which goes a long way to making more controlling decks in those colors work. Wrath of God and Day of Judgement i'd definitely slot in. You also don't have Banishing Light which while being yet another O-Ring is still a fantastic card. Supression bonds is just a worse Faith's Fetters. I also would look into getting a copy of Swords to Plowshares which is pretty inexpensive and one of the best removal spells printed.
There are a bunch of other things that come to mind but for starters that's where i'd start. Just how much of a budget do you have?
I liked Architect it just has the downside of being a 4 mana card and having stiff competition as well as being a planeswalker. Then again I still run Tamiyo at 540 but that's more of a pet card at this point than anything.
I'm not a fan of cutting him but I can't remember the last time I saw anyone besides me play him. This card is something I can easily see multiple people in my playgroup be excited about so it gets the nod.
I think this card will excite players in my playgroup more than Greater Gargadon currently does for my group. While i'd rather keep the Gargadon I'll probably swap it out for this because I know people will actually pick this and play it in my group.
I just dropped one of the 4 drops and this doesn't look any better than the 4 drops I'd want to include/re-include if I wanted another in gruul. Cool card but not good enough.
Omnath has been really solid in my cube. He either dies very quickly or things get out of hand real fast for the other player. Definitely high variance but he has a really interesting ability.
If real this card looks quite interesting to me even without the last ability being easily obtainable. I'll have to cut Unburial Rites at 540 for her but that isn't too big of a loss. She isn't Souls/Sorin, LoI/Vindicate and I am not sure I like her more than Magister either.
You have a grand total of 4 slivers in the entire cube. I'd cut all of them for somethin more universally applicable because as is they simply won't matter often enough if ever.
There are a bunch of other things that come to mind but for starters that's where i'd start. Just how much of a budget do you have?
Chained to the Rocks vs. Slayer's Stronghold vs. Assemble the Legion vs Gisela, Blade of Goldnight
Reckoner always ends up sideboarded so I want a new 5th card in Boros.
List for Refrence: http://www.cubetutor.com/viewcube/23816