After some extended time with 3X and 5E, our group is returning to a hybrid of 1E and what we like from 3X and 5E.
One of the things that was specifically asked for was that we play be the rules that require training to level up and/or change classes. So, we're planning on sticking as close to the AD&D DMG guidelines as possible (with some changes in pricing as we also decided to change the economy to a silver standard).
We're likely keeping XP for gold pieces found, but with the reduced treasure rates, it will have less of an impact than in AD&D RAW.
I've also revised the XP for encounters based on the average party level (rather than monster HD or calculations based off of HP and special abilities) and a multiplier for whether the encounter was easy (x0), normal (x1), average (x2), hard (x3), or deadly (x4). Easy encounters grant no XP, while deadly might result in a near TPK, even with good tactics (and definitely some PC deaths). This portion is based off of the XP rules from Dungeon Crawl Classics. (I'd originally wanted to drop the XP charts and use the DCC RPG unified chart and awards, but the players wanted to keep the individual class-based XP charts, so this is the best of both worlds, for me.)
The main goal here is to have slower advancement than we experienced in 3X or 5E. We're looking at about 25 average encounters (for a fighter; other classes will vary) to get from 1st to 2nd; 30 to get from 2nd to 3rd; and increasing by about 10 encounters needed per level from 2nd to 3rd and beyond (from 19th to 20th would take about 200 average encounters for a party of 19th level characters).
Interestingly enough, XP rewards peak at name level (around 9th) and then start dropping off. So, even though a fixed amount is usually needed from about 9th level on, XP awards start dropping off and advancement slows.