The first two Paper Mario games are fantastic. I love them both dearly.
If I had to choose between a remaster or remake of either, I think I'd go with the original, as much as I think TTYD is a better game. I think the first game has more to gain in the audio-visual department. Not only that, but if Nintendo wants to introduce the proper games to a new generation, they might as well start with the first one, and then follow-up with TTYD some time later.
Outside of the audio and the visuals, I would very much like for Paper Mario to have its combat upgraded to get the audience from TTYD. I thought that mechanic was super cool and added style and atmosphere to the game. It would work perfectly in the first game as well.