I haven't spent more than 1 minute thinking about this, but I'll just leave this here.
You know Shadowrun? The crappy counterstrike-esque magic-meets-fps shooter for 360 and PC? (Actually, it was pretty decent, but the game was gimped by console development and crossplatform multiplayer).
As you would except from a counterstrike-like game, you buy weapons. But you could also buy abilities after every respawn. (It worked with rounds)
This included buying a healing AoE 'spell', revive, create spike traps, a sort of glider, increased speed with a katana, et cetera.
Every player started the match equipped with just their race's traits and either a pistol or smg, but by the fourth or fifth round, some people were using a dwarf sniper + healing/revive spell while others were going for the rambo approach with a troll minigunner + lock-on tech. Quite possibly, opponent's team would then react by going for elf katana + teleport.
Now, your shooter probably won't be rounds-based, but this got me thinking if you couldn't make it so that players get to pick certain 'build orders'.
Say, in a ten minute match:
- After 3 minutes, the first tier upgrades unlock. Players press a quick hotkey or w/e to select which upgrade they want, and these stay the rest of the match. These powers are pretty minimal in their gameplay effect.
- Three minutes after that, players pick another upgrade. These are a bit stronger, similar to Sine's idea. Again, these stay for the remainder of the match.
-Two minutes before the end of the match, third tier upgrades can be picked and these are quite powerful.
This would make it so that players don't pick a class before the match, they create a class during the match, based on their assessment of the opponent.
Both players can pick the same upgrades if they want, so no restrictions there.
The trick in making this work, however, is to make it so that one certain combination of upgrades is not always the best, not even on one particular map.
I'd imagine it would be fun if you could stack upgrades towards a certain goal, but each upgrade also came with a weakness. (e.g. 1st: higher damage on splash weapons, 2st: projectiles move a bit faster, 3rd: RL charges similar to UT's alt fire. Not the best example, but yeah...)
The weakness in this case would be reduced movement speed or something, which would make you more of a target for hitscan.
That would then be one 'maxed' build, but you could also opt to choose upgrades from several 'chains', to spread your chances of success.
I dunno, but I would appreciate something like that. Maybe not for 1v1, but for 3v3 or 2v2 TDM it could be lots of fun, given that your options are indeed open.