I played Rumble last night for the first time with a group of people
who, like myself, are former MtG and DnD players so therefore have a
pretty decent sense of balance.

Here are the powers we came up with, if the wording is too ambiguous
on them I can clarify, as all of the powers were used last night and
the creator of each power was present.

Siphon Strength
Use only once per round.
During your allocation phase, pick a Hero. If that Hero allocates any
Energy points to Attack this round, gain Energy equal to half of the
allocated total. (Round up)

Nuclear Bomb
You may activate this on any round at any time. Once active, the Bomb
is an attackable object with 10 Defense and 50 non-replenishing
Energy. At the end of every round, put a counter on Nuclear Bomb. If
at the beginning of any round there are 6 counters on the Nuclear
Bomb, it detonates, and all players, including the owner, lose.

Experiment Gone Awry
If your Energy level falls below the Power Strength of this power,
immediately lose 5 Energy and gain one power at Random from the
deck/archive, then discard Experiment Gone Awry.

Brilliance
Any attack that does 15 or less damage to you does half damage. Round
down.

Prismatic Field
Spend X, Burn Y: Increase your Defense by X for Y/2 rounds. Round Y/2
down.

Last Ditch Effort
Use only once per game, and only if you have the least amount of
energy in the game. Halve your Energy level:

Triple the strength of all your Attacks for the duration of the game.

Probability Shield
Spend 10, Flip a Coin:
If heads, all attacks against you do zero damage.
If tails, halve your defense for this round.

Decoy
When a hero attacks you, they have a chance to hit your decoy. Roll a
d6. If it is a 6, the attack is successful. If it is 1-5 your decoy
is hit instead. Your decoy can be hit five times before being
destroyed, regardless of damage amounts.

Super Speed
Burn 5: One attack you make this round is unblockable.

Calculated Strike
Spend X, designate a Hero:

That hero is damaged for 2X damage during the next round's resolution
phase. This damage is unblockable. This power may not be used on a
turn in which you allocate any energy to attack.

Mortal Wound
If anttack made by you damages a Hero for half their total Energy or
more, they are mortally wounded. After 2 more rounds, they are
eliminated.

Energy Source
Add 10 to all Attack and Defense allocations you make for the duration
of the game.

Brute
Burn 5: All attacks you make this round are doubled.

Chaos Pendant
Roll a d6
1: You take 5 damage
2: Swap Chaoes Pendant with any other power in play
3: All opponents take 10 unblockable damage
4: Choose an opponent. All attacks made by and against that opponent
are multiplied by 1.5 (rounded up) for the rest of the game.
5: You gain +10 defense for the rest of the game.
6: Double your Energy and discard Chaos Pendant.

Disorienting Punch
If one of your attacks does more than 10 damage to an opponent, they
are disoriented. That opponent cannot make any attacks next round.

Wall of Flames
Spend 10: Any hero who attacks you takes 10 damage this round, 6
damage the next round, and 4 damage the round after that. You take 2
damage for each of those rounds.