I'll stay with suggestions, since I don't expect you're going to change the tier system after putting so much developement into it. So here's an idea to improve it.
Currently there are only 15 valid end game ships. Once you get all Tier 5 there is no reason to ever play anything else.
So add one unique Module to every Tier 4 ship. Something not available on the Tier 5 version. Sort of an "Experimental variations". These could be new unique modules or even off class modules that would make the Tier 4 version an appealing alternative to the Tier 5. This could even attract people into a tree of research they were not interested in. If you told me there was a Tier 4 Destroyer that could have a Bomber Bay, I'd definitely check that out, and once fully invested in it, I'd pick up the Tier 5 also even if it did not have the bomber bay, just to have more options.
You could also use this as a "Rock Paper Scissors" sort of fix, offering Tier 4 versions that could be strong counters to certain Tier 5 ships if you chose the right match up. I'd love to see a completely Anti-Corvette load out for a Tier 4 destroyer, that could be chosen when a corvette player is dominating.
Doing something like this could expand your end game ship variety to about 30 ships.
If you had time, you could even do this to Tier 3 ships as well.