I'm currently designing a rough prototype of a mecha fighting game.

These are the basics I came up with:

• Multiplayer (matchmaking for up to 10 people, for now)
• Browser based (HTML5)
• 2D (<canvas>)
• Persistent (as in, players have accounts and don't have to use a new mech each time they start a match)
• Players earn money upon destroying another mech, which is used to buy parts (guns, armor, boosters, etc)
• Simplicity (both of the game itself, and of the development of said game)
• No "leveling" (as in, players don't get awarded with XP)

The last part is bothering me. At first, I wanted to have players gain experience points (XP) when destroying other mechs, but gaining two things at once (money and XP) seemed to be in conflict with my last point, which is simplicity. If I were to have a leveling system, that would require additional development. But, the biggest problem is that I simply couldn't fit it anywhere! Adding levels would require adding meaning to these levels, and most of the things that I hoped to achieve could already be achieved with the money mechanic I introduced. So I decided to drop leveling off completely.

That, in turn, removed a fairly popular and robust mean of progression in games from my game (not that I would use it well anyway). Is there another way of progression in games, aside from leveling and XP points, that wouldn't get rendered redundant by my money mechanic, would be somehow meaningful (even on a symbolic level), and wouldn't be in conflict with my last point, which is simplicity?