An interesting topic, personally I'd be all for new ships (new Nepthys model please?)
Baconsoda however I agree with, with the exception that there is precedent for the introduction of new aircraft in the life cycle. Take the F22 Raptor (even with its problems) for example.
Secondly, I would add the example of the F/A-18 Hornet. It was introduced in 1983 and now is known as the F/A-18 Super Hornet, sooo
From this perspective the logical thing to do would be to showcase the guardian as a "superhornet" or upgraded defender, where as maybe give liberty an entirely new fighter to represent the introduction of new systems and what not.
So therefore, keep the current mark IIs, give every house faction a new second generation VHF
EDIT: I must correct myself, the Super Hornet is a complete redesign of the hornet and is apparently bigger than the original Hornet (according to wikipedia)
Spot the difference? Its only small but its there, this is what the op is talking about, small changes that show the ships are improved versions of their predecessors rather than carbon copies with larger numbers in their ini.'s
I am for these small aesthetic design changes, but if the vanilla models where brought up to standard and the mkII's removed that would be cool as well. We'd still have civilian ship-lines for those who want to be challenged by low lvl missions.
edit: found a better perspective of the super hornet