Without the tuck rule there would be no point in designing a deck around a commander. Where's the fun in not having your commander in an otherwise "fun" format? Instead, which is what people did, they would play "pure value" commanders and then have a deck that doesn't depend on whatever legend or effect you're going for, whatever that may be.
If I had a cube that wasn't a pauper cube I'd not run any planeswalkers. I don't like how they play and the game would be better if they weren't in it.
If you want to encourage monocolor play then avoid hybrid and play all landtype matters and color matters type spells. Make your bombs stuff like Dawn Elemental that costs WWWW or all of those demons that have BBB in their casting cost. Also monocolor tribes, perhaps? Put in a bunch of goblins but only the red ones or zombies but only the black ones.
Cut pretty much all fixing. So no duals or mana rocks except for ones that make only one color.
Favour removal and other bomby spells that cost double color. So wing shards instead of pacifism.
Processor was often the target in the good old days of Urza Saga T2 since you could just pay 10 life or something and KO the opponent. In a cube with more answers to artifacts paying 4 and getting a 4/4 every turn is still pretty brutal.
Colossus with Voltaic Key was also popular since you avoid the whole eight life thing.
5 Mana is a lot for 2 multicolored power. It Depends on how easy it is to kill a 5 toughness unblockable creature in the cube. If there's a lot of efficient answers then its probably not worth it when there's so much goodness in blue and white. There's several 2/5, 3/4 and 4/3 flyers for 5 nowadays and flying is not that much worse than unblockable if you have enough kill or bounce.
If I'm not going to get infracted - Tidal Wave is a fun sort of card, but it's basically a kill spell that says "destroy target creature with 5 toughness or less if your opponent does not discard a removal spell". Blue has way better ways to remove creatures than that than a super situational kill spell. It's too bad that the token has Defender, so its not a blue Waylay. They totally missed the opportunity to allow the wave to "crash" over your opponent if you cast it at the end of the enemy's turn so you get a combat phase with it.
Deftblade Elite is amazing. Early it can trade off strong utility X/1s like Undertaker or Fireslinger. It also goes super nuts with auras or equipments. Drawn later in the game it's a brick wall that can roadblock every ground beater.
Soul Manipulation is super strong. Dimir is stacked as hell, but this is definitely a valid option. It's a straight up 2 for 1, that also gives you the card selection of choosing whatever creature you would like to return.
Apropos Prowess in general (e.g. Sanguinary Mage), it's simply a really bad mechanic. To trigger it constantly it often requires you to sequence your spells badly. Usually you want to play your instants an sorceries at the latest possible moment (2nd main or your opponents end step) and keep mana open to deny your opponent the information what you might have in hand. Even though theoretically it sounds nice to use any kind of noncreature spell as a kind of free combat trick there are actually very little spells you want to play during combat just to get a tiny +1/+1 push.
It doesn't sound that helpful, but my best advise is to simply use cube-comparison-tool from cubetutor. Simply compare your cube with updated lists from other people (link in my signature) and see which cards no one else is running and think why that might be and also see good alternatives.
Fireslinger can just tap in response to the untap and kill elite. And if you really want undertaker not to die you can also just activate its pitch ability. The way Provoke is worded any utility creature that taps to do the utility can always escape being killed.
If you want a 1-drop that kills other 1-drops, javelineers is way better.
No shroud, sadly, but effectively unblockable vs non-red decks with a bonesplitter is a nice clock.
Cut pretty much all fixing. So no duals or mana rocks except for ones that make only one color.
Favour removal and other bomby spells that cost double color. So wing shards instead of pacifism.
Colossus with Voltaic Key was also popular since you avoid the whole eight life thing.
5 Mana is a lot for 2 multicolored power. It Depends on how easy it is to kill a 5 toughness unblockable creature in the cube. If there's a lot of efficient answers then its probably not worth it when there's so much goodness in blue and white. There's several 2/5, 3/4 and 4/3 flyers for 5 nowadays and flying is not that much worse than unblockable if you have enough kill or bounce.
If I'm not going to get infracted - Tidal Wave is a fun sort of card, but it's basically a kill spell that says "destroy target creature with 5 toughness or less if your opponent does not discard a removal spell". Blue has way better ways to remove creatures than that than a super situational kill spell. It's too bad that the token has Defender, so its not a blue Waylay. They totally missed the opportunity to allow the wave to "crash" over your opponent if you cast it at the end of the enemy's turn so you get a combat phase with it.
Fireslinger can just tap in response to the untap and kill elite. And if you really want undertaker not to die you can also just activate its pitch ability. The way Provoke is worded any utility creature that taps to do the utility can always escape being killed.
If you want a 1-drop that kills other 1-drops, javelineers is way better.