More cleric domains
Dungeon magazine
A better index
A demonstration of how to fill the character sheet.
As for the argument;
Generalist wizard = don't take a school. Bam done.
Skill monkey rogue or ranger = ask your dm to swap sneak attack or spells for more skills. Done
This. I'm happy with 5E's basic system as it is, but a really crunchy optional tactical module could bring in some of the people who haven't jumped in from 3.5/PF or 4E so far.I think 5e needs a few more modular options that support a wider range of play-styles. I was honestly expecting a lot more in that department.
I've had people who were big fans of 4e tell me it was a tactical board game in essence.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.