I found it a lot easier to build a trade nation in Medieval 2. I played English and first thin I did was abandon Caen and retreat to the British Isles. Then I just deployed diplomats to as many capitals as I could. Slowly I gathered 500,000 gold, built up a huge army and slowly worked my way across the continent. Anytime I couldn't just roll over an enemy I besieged their cities then demanded peace. It worked really well.
...Until I killed the Pope. Didn't last long after that.
Trade was confusing in Med2. I seemed like the more nations I established trade with the more random my trade income would be. Like if I played Sicily and set up trade with the Venice, Papal States, Genoa, and the HRE; the second I set up a sea route all my land income would vanish. I would just end up sacking capitals and letting it rebel so I could sack it a few years later or ransom it back then sack again.
Sounds like my campaign as Holland in N:TW.
After years of fighting i've gained the upper hand though, and dominate the high seas.
Pretty much every nation hates me but they can't fight me because i earn 15000 ish from sea trade and they earn 0.
In Empire it's fairly hard to establish a formidable fleet if you aren't a major nation. Like my Maratha playthrough I could never make a fleet as the AI always shows up on the second turn and blockades the shit out of you. Ontop of that France declared war on me because I wouldn't trade land in India for land in South America. Then they raided the shipping lane off of Portugal killing all of my trade income.