I'd like an even more interconnected world. The early-game areas are amazingly interconnected, but this trails off when you go after the Soul Lords and end up going down linear paths. Actually, the end-game areas as a whole aren't too good compared to the well-designed castles with treasure and shortcuts aplenty. The only exceptions are the Catacombs and the Duke's Archive.
A more lenient difficulty curve at the beginning would also be good. Demon's Souls does much better on this front, since it allows you to get used to the combat controls before pitting you against the harder soldiers and Blue-Eyed Knights. Dark Souls just dumps you straight at a boss without much indication that you're supposed to escape from it, and when you get round to defeating it you won't get much practice in for fighting the first real boss, the Taurus Demon.
That said, Dark Souls does much more right. The world feels much more lived-in and detailed, whereas in the first game you jumped between worlds willy-nilly and never really got an idea of what they were all about. Thebonfire system is also much better than having to go back to the Nexus whenever you wanted to store something or smith/repair your weapons. Having a limited supply of healing items is also annoying in Demon's, since when you run out you have to run through 1-1 a few times to get more or buy them from merchants, and you can just grind through bosses by endlessly nomming Half Moon Grass.
If FROM do make another Souls game, it should have more elements from Dark than Demon's.
Also, fuck the dragon bridge in 1-2. Seriously.