Goku turns "Super Saiyan" battling Frieza.
Lex Luthor drinks an elixir to assume Kryptonian powers.
Tony Stark activates "Iron Man 6".
Bane injects Venom to bolster his strength.
Heroclix and Malifaux both feature a "Level Up" mechanic, I don't know much about Malifaux, but the 10th Anniversary Heroclix sets had some neat ideas. Each character that could power up had two separate Clix in the vein of Marvel Girl and Dark Phoenix, Weapon X and The Wolverine, Grey Hulk and Worldbreaker, and so on. The "weaker" character and be swapped out for the "stronger" when specific conditions are met, which differ with each character. In a game like Heroclix, I am inclined to call this just another tactic to be used to win games, or cheat expensive characters onto the table, however, for something like SuperSystem, it could be a bit of fun.
For those familiar with my Narrative Campaign (http://www.lead-adventure.de/index.php?topic=32726.0), because I am modifying the [SS3] rules drastically, creating characters that "Level Up" is reasonably straightforward. I've already attempted drafts of the above Goku and All-Star Lex Luthor characters that I'm waiting to playtest. (For those curious, Goku can transform at will, but Lex must be losing to play his trump card.)
The SuperSystem "Alternate Form" power looks like a tempting option, especially when one is considering an actual transformation (Super Saiyan, Hulk, etc.), but my reading of the power, coupled with how Scott P has written about it makes me believe that it represents staying on par more so than getting bigger and stronger. To put it in SS3 terms, what I'm thinking of is having the opportunity for an 85BP hero to become a 100BP character during the course of the average game. Not every character, perhaps as many and one per team of five, and certainly not in every game, but in a campaign, it could spice things up a bit.
I have a few ideas on how something like this could work, I'd like to hear what other players think:
The [disappointing] Super Smash Brothers Brawl introduced the Smash Ball, an item which can turn any character into an unstoppable killing machine for a brief period. Whilst this makes the game awful to play, the idea of an item allowing such a power-up isn't necessarily terrible. In comic turns, something like a Cosmic Cube, an Infinity Gem or even a Super Soldier Serum could give a character an advantage immediately. The primary problem I see with this option is that it gives movement-based characters too much of an advantage.
Three seconds before someone throws a controller through the screen.
An idea that may be a little more fair, what if a hero can roll themselves bigger and better? It may have to be different for each hero, but say, for example, a goal-only roll (no 1s, 2s or 3s) could cause a level-up.The idea behind this option is that a Hero is doing so well, that they become essentially unstoppable. The problem with this option is that more than one of my regular opponents roll terribly, so they would see this as unfair.
Green Lantern Batman. Because Batman wasn't scary enough.
3. Last Legs
The final option I came up with was a hero being able to get stronger when only on a few Hits. Something along the lines of a hero putting everything they have into a last punch. Balance wise, it seems to be the best option, as players have to play cagey to get the right amount of damage or just roll with it if it happens.
You fool! You only made the Punisher mad!
So those are my thoughts, keen to hear what other people think.