As we played yesterday, a few guys joked about improving the ships.
And as we have right now no "flipping option" on the backside of the
ships, I propose a two level ship. Usual Ships can be improved to
Warships. It is right now in the idea phase, but
maybe somebody is willing to discuss.

The Idea on scratch:

Normal Ship - Frontside of Coin
Ship Building: 2 token
Ship Maintenance : 1 token each round

Warship - Backside of Coin
Warship Building: 2 (or even more) additional token,
Normal Ship will be flipped over with a Warship Picture on the back of the Ship Coin
Warship Maintenance: 2 (or even more) token each round

Warships should have some additional capabilities like
- blocking coastal areas for other ships (but not open sea areas)
- transporting more people then 5
- moving slower, as they are stronger and heavier
- warship may hold troops on it, these token stay on board
and are not counted for city support, but for census, they are
not affected by Population Expansion
- Warships may attack each other, including the troops on board

Maybe this should be allowed only after buying "Naval Warfare"?

Any Ideas on this? I know it is way to complex for the base rules,
but I think about something as optional rule "Naval War" for the
optional rulebook. I just would like to have some more ideas before
playtesting this...

Oh yeah, this is a fun idea, but maybe better put this in the variants area of this forum and put a link from here to there.

Placed there also, can somebody provide the link?

Here you go:

http://www.civproject.net/forum/viewtopic.php?t=527

Something like this might work for the "Rise & Fall" variant I've been kicking around.

Any links to that??

Nope - its all just in my head for now. I'd like to do a varient where the empires are much less static (and hence more historical) - where players can leave empires that are doing poorely and new ones can rise up organically. I'd also like to factor in things like armies and navies (but keep them abstract).