I think I could make this game work as something that is appealing to me, by creating my own custom XP award system to roughly double the time to level up, using the slow rest variant that requires a week of rest to regain spells, enforcing food and water mechanics, overhauling Encumbrance, hard-capping the game at 10th level, porting in the wandering monster mechanics and monster reaction rules from older games, and so on. But would that even still be running 5th edition or rather some custom homebrew abomination?
I guess this question is primarily directed at people who consider 5th edition to be a system that works reasonably well enough for a more old-school style campaign. What exactly is it about the mechanics that has a certain old-school ring to it, or makes it suitable to be used for such a purpose?
@Yora It all depends what you are looking for in that "Old School" feel. Old school means different things to different people. Also, some people don't play OSR type games, because they don't like or want some of the Old Schools things. We started out playing D&D in the 80s with a mashup of AD&D 1e and BECMI, so we have a bit of that Old School mentality. We have several house rules that give us an Old School feeling (and some that do not). They don't break our game and it is very much still 5e (as in you can use the monsters in the MM and official adventures to some extent). From what I hear from you, here are my suggestions for you:
Leveling: It took my group 6 years to reach lvl 15 playing 1 or 2/month for 4-8hrs. How did we achieve this, we ditched XP completely. Now we did this back in the 80s too, so it was an Old School feel for us. But e simply level when we feel it is appropriate. Alternately, if you still want to track XP, just make the requirements for each level higher as you suggest. However, I might triple the requirement, not double. One leveling house-rule we have that you might like is that we can only level up during Downtime as you need to research, train, etc.
Ability Scores: We cap scores at 18; however, if you want a really Old School feel cap them at 16. 5e still works very well (I say even better) with lower ability scores.
Hit Points: Instead of capping level (because you want those features down the line), soft cap hit points at level 10. Then, starting a 11th level you get fixed hit points based on your HD only: 1 for d6, 2 for d8, 3 for d10, and 4 for d12.
Rest and Recovery: Keep short and long rest as is (for spells and abilities), but add Extended Rest. An extended rest is one week and must occur in a safe place. You can send 1 HD to recover hit points after each extended rest or 2 HD if you use a healers kit.
Death and Dying: Option 1, death at 0 (this is what we do, but we have other non-old school rules the affect this). Option 2, every time you make a death save you suffer a level of exhaustion. Option 3: no death saves and your dead at negative hit points equal to your CON modifier.
Food and Water: We didn't use rules for this in 1e and we don't now. However, I have heard good things about how Level Up (Advanced 5th Edition) has tools to enforce this. You might check out this thread:
Reflections on Exploration rules in play for how they give that type of Old School feel. It seems right up your alley.
Encumbrance: We never used it 1e and we don't now. I don't really have any suggestions on this one.
Magic:
- Limit cantrips. Option 1: They require a spell slot to cast. Option 2: you can cast them prof. bonus /long rest.
- Spell disruption: If a caster takes damage during a round, before casting a spell, it can't cast that round.
All of these can help give the Old School feel and work very well with 5e. In fact, I think the default game actually works better with revised ability score caps and hit point rules. It makes Monsters more threaten, but not out of reach, at higher level which is a problem some groups have.
The big issue you will have is official adventures if you use them. You will need to add a lot more content to level up your characters to the expected levels of the adventure.