how bout you have it as you start out as an imperial guardsman and after many battles, enemies killed and experience gained a chapter or the emperor grants you the chance to partake in special space marine training, where as you get massive stat boosts and the famous space marine armour and weapons, or if you dont want to become a space marine you could continue on as an imperial guardsman into the higher ranks, such as a commisar or a karskin(sp?).
The same could work for other races, such as the eldar, you could work your way up from a lowly guardian and at certain levels or something you could be offered training in a certain aspect, e.g Striking Scorpions or Warp Spiders or Banshees or anything else, and maybe at the level cap or very high levels even train to become a farseer or warlock.
To stop the ganking of low level players they should level lock planets or certain areas, sure it would be annoying for some people not being able to go to that new planet and partake in a massive battle against a tyranid splinter fleet but its less annoying than getting there and dieing like 20 times and doing no damage to any1 because of their higher levels than you.
What do people think of that idea?