Having triremes that can go on rivers is an excellent addition to the ruleset. So useful for moving stuff around within your own lands or invading others.
Unfortunately the possibility of building triremes is lost after Engineering, and if you have Leonardos workshop your existing triremes will be uprgraded to transports. So the ability to sail up rivers is lost when your ships become more advanced! ???
So, I think it would be good to have a modernised version of the trireme available, that can be built after researching Amphibious Warfare.
Unit Name: Landing Craft
A 0 D 3 HP 30 Move 10 (So same defence as transport but less moves)
Still only coastal but able to move on rivers.
Transport capacity still only 2 or 3, but able to carry all unit types, including big siege and mounted (Armor).
Could use the Ironclad icon, as this will already be obselete.
Triremes would upgrade to this unit.
Not a bad idea.
Would it be understandable enough if the new and enhanced trireme would use the same graphics as the ancient one? I would prefer to not use ironclad graphics for a new trireme because it may be possible to tweak the ship settings to create a reasonably good and usable ironclad. One way to do that might be decreasing the attack of a frigate to 3 and keeping ironclad's attack at 4.
Or... we could add a 2nd transport unit with identical graphics to the transport. The difference would be that this unit could travel on rivers, would have less moves and could not travel deep ocean.
And for LT37, there is one way to prevent Leonardo from upgrading the triremes. If a trireme is outside a city and on a river tile, Leonardo will not upgrade it because it can't be upgraded on a tile where the upgraded ship can't exist.
For the graphics, please define something like
graphic = "u.barge" graphic_alt = "u.transport"
for those of us who might like to use a specific sprite for the new unit (sounds like a barge to me ;-)) in their personalised tileset.