How about this: In First Edition PnP, the XP charts went up to 20 and then said something like "1 million XP per level hereafter, +1 hit point, etc" except for Monk and, If I recall correctly, Druids, both of which had hard caps (Monk was capped at 15)

Instead of a set number of XP to level, allow unlimited levelling, but the XP to go up every level past 20 is double the XP for the previous level.

Problem Solved. First one to level 100 gets to watch the sun burn out.