Just adding more ship types would be great. Besides the Karve and the stereotypical Longship we already have, there were also Knarr- cargo ships as opposed to warships. Beamier, more freeboard, more cargo capacity- but slower.
And Longships came in all sizes, from a "seven bencher" (seven pairs of oarsmen) to the 16 bencher Gokstad ship (actually a Longship-Knarr hybrid) up to King Olaf Trygvasson's "Long Serpent." That ship was a monster. She was a 34 bencher, and built to the Knarr plan for better seaworthiness and to be a better battle platform. She was the last ship taken at the Battle of Svolder in 1000 CE. King Olaf's body was never found; according to the story, either in a blind panic or by design, the king decided to go swimming with his armor on.
The Drakken Harald Harfagre (Harald Fairhair) is a pretty impressive beast in her own right. She's a beamy 25 bencher, and is currently sailing the world. She had a crew of 34 on her last American tour, but she's capable of putting two men on each oar, for a total of 100 oarsmen.