This is extremely similar to an idea I had the other day:
a) 100 XP to level up, regardless of your level.
Yup - Mike Mearls had the same 100-XP idea in an Unearthed Arcana column, and that certainly influenced my thinking. I played around with 100 and with 20 - I'd use 20 if I were going for 5 sessions to level, just as Classic RC D&D recommends 5 sessions/level and quest XP awards of 1/20 a level. There's also Savage Worlds which uses 5 XP per Advance and 2-3 XP per session. I also considered pure fiat or by-sessions leveling. But for what I'm going for, and to encourage swifter advancement, I think 10 XP based on the 4e standard of 10 encounters/quests per level looks best.