Well Babylon 5 without proper ships is kinda...weird. It would be missing something.
Anyways, yes, i have been thinking about a concept. I assume the mod will be heavily influenced by the original SEIV one. This means most pictures for the weapons (even at this point, excellent doga work) will be used, same will LIKELY be for reactors and armor (pictures that is).
The armor system will very likely be something where armor plating will soak up a considerable amount of damage depending upon the tech level. In addition, higher level armor will likely be able to negate certain damage (think leaky/emmisive if im not mistaken).
Due to the pure amount of weaponry (SEIV mod) i would like to realize something where players can research small and big guns and small guns would be far less effective against higher level armor (i.e. armor absorbs 15 damage (fixed value) and consumes 80 % of what is left, small weapons will do 25 damage, big guns will do 100 damage).
Im basicly thinking about "weak" pulse weapons and "strong" beam weapons.
Engines is a difficult thing. QNP is pretty obvious. I will definitly not consider adding scrips and/or hyperspace. While it my lack flavour, you will still use existing wormholes and there will be certainly be no wormhole generator on every capital ship. NO
Doing what you suggest may be possible (i dont know) but way way way too much work.
Shipsizes, i dont know. Mainenance is the key of course. I still like the "stock" system though, researching bigger hulls and fixing them via weapon/armor mounts sounds reasonable. Maybe im too attached to SE IV ?
I as well plan to play a bit with facilities. Instead of having each facility be sized 1 kT, i might do something where research facilites (aka) will be sized 2-3 kT or even 1.5 kT to make planet development a bit less of a no-brainer.