Hive Stirrings for Lingering Souls
Master's Call for Raise the Alarm
Welkin Tern for Stratus Dancer
Air Elemental for Djinn of Wishes or Murder of Crows
Ojutai's Summons for Talrand's Invocation (technically not a strict upgrade, but pretty close since you don't have any prowess/"spells matter" effects)
Festering Goblin for Shambling Goblin
Severed Legion for Flesh Carver
Dragon Fodder for Mogg War Marshal
Viridian Shaman for Reclamation Sage (Not really strictly better, but close enough. I'd run both anyway.)
All Guildgates for KTK taplands (Rugged Highlands) or KTK/Shards of Alara Trilands (Frontier Bivouac) - Although you might want to up the amount of mana fixing in your cube. A common rule of thumb is three sets of dual lands per 100 cards in your cube plus as many 5 color fixing lands as you can get your hands on, especially ones that don't come into play tapped.
I'd also like to share this list of budget cube staple cards that was very helpful for me when I first built my cube, and I think you might enjoy trying out. The list is about a year old, but it has a lot of great cards that don't cost more than a buck or two that are very efficient and versatile. This could be a good starting point for upgrades if you'd like to try out some newer cards that have stood the test of time in a lot of cubes:
Finally, I'd like to invite you to try out the "Draft the Above Person's Cube" thread here: http://www.mtgsalvation.com/forums/the-game/the-cube-forum/488984-draft-the-above-persons-cube?page=154#c3856. It's a really fun way to check out other people's cube lists by drafting them and it also helps you get helpful data on the most popular and unpopular picks in your cube based on the kinds of decks other users here draft from your cube. You can see top and bottom picks in the "Draft Charts" section of the "Analysis" tab of your Cubetutor page. Before you do though, you should probably fix the duplication issue in your cube list.
I hope all this helps, and Happy Cubing!
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Some other cards of note I found that fall in the prison line Authority of the Consuls which slows down your opponents haste creatures. Hushwing Gryff seems like it could fit in very well along with Aven Mindcensor. Protection of the Hekma probably won't make the list because of the mana cost and Sandwurm Convergence (unless you have a very slow meta). Thalia, Heretic Cathar also seems nice in a prison strategy.
As for finishers some I think that could work are like Dragonlord Dromoka . She's just a big body that also locks down your opponent on your turn. Hornet Queen also could possibly work with good ramp to back it up. Some planeswalkers could also fit or even Approach of the Second Sun if you wanted to go that route. I think the deck as a concept sounds fun though and entirely possibly, just a little more difficult to brew with.
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Hmm looking at this list you could quite possibly make a budget tezzerator list. With your budget you may have to do some interesting. things but you could either go UB if you are willing to get new Tezz and maybe a few other artifact toys, or heck even mono U if you are willing to buy Vedalken Shackles. I used to play the deck in the past and it was a lot of fun. The bridges and crucibles will work very will in the deck, and it looks like you have a good number of good blue stuff to help out.
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You're saddened that we as a playerbase agree with banning someone who acts like an ***. Well then.