On topic: IMHO If the reward is not a gameplay (meaning it visually and obviously changes something in-game) item, then it doesn't matter. See expectations of many newbies of "upgrades" or "new weapons" as rewards for level ups.
For visual and obvious changes, look around at your crewmates, and view yourself in third-person for comparison. See skill level comparison between "newbies" and veteran players for reasons why actual gameplay-related alterations would be a bad idea. Without new content, skilled players demolish inexperienced crew members. I'm still new, but I've picked up a lot of things very quickly. I'm not quite one of the big kids, but I can hold my own in random non-novice pub matches and usually finish on the winning team.
For gameplay items that are not game breaking I'd suggest filters to make the game saturated/black and white, different weapon themes/SFX, voice packs. Anything that feels like an "upgrade."
Filters is actually a cool idea, but not gameplay-related by any means. Purely cosmetic, like character customisation. Not sure how viable an option it would be for the game as it currently stands, but if possible, I'd actually quite like this.
Weapon themes are something for ship customisation, and would be cool, BUT the idea of changing weapon sounds is kind of problematic, imo. Hearing is an important sense for gamers, and compromising it could weaken gameplay. Being able to quickly recognise the weapon you're being shot with makes a huge difference sometimes. Not saying it can't be done, but each weapon's sound would have to be distinctive and still similar to the defaults.
Character voices as later-level unlocks though? GREAT! I'm all for it.
EDIT: You may notice the only suggestion I'm disagreeing with is also the only one with an actual impact on gameplay.
