I'm currently advocating a system whereby rank is separated from level, and bound to 'rank titles' instead - see this thread, here...
The benefits as I see them are as follows:
1. It allows players to choose their ranks from those they've unlocked, so players wanting to stay at Captain can do so, while simultaneously allowing Admirals to stay at 'Admiral' if they wish.
2. It'd provide an opportunity to fix the clunky 'Rear Admiral, Lower Half' NPC dialogue to more accurate versions.
3. It'd mean that any future levels wouldn't need to include increasingly ridiculous ranks - we don't need EVERY player at 'Fleet Admiral' rank, and what's next? 'Commander-in-Chief'? 'Federation President'? Separating rank from level allows for unlimited level-cap raising, without such problems.
Any solution that continues to tie rank to level as the OP proposed doesn't fix the problem. It just postpones it.
How about seperating the leveling system from the rank system. Meaning you can level up to 51 but still be a Lieutenant however a higher ranks lets you access better tiers of ships. How about tying the rank system in with the accolade points