I'm wondering about possible rules for using downtime training to develop certain feats, instead of/in addition to swapping ASIs for feats. Has anyone done this? Any guidance on how to work out the math/requirements?I see that the Skilled feat lets you choose 3 skills/proficiencies, so I would suppose tripling the costs of training for a skill would be one approach.
IIRC to learn a
tool or
language it takes 250 days of downtime. If you equate a
skill with them and a
feat to 3 skills, then you get about
2 years to learn a feat.
But in addition, chapter 8 of the DMG says you can let the PCs learn a feat (or skill) as a
reward for a quest, rather than standardize how to get more skills/feats than the ones you get from levelling up.
Consider that those times for downtime activities in the PHB/DMG are meant to be reasonable from a narrative point of view, but are impossible to "balance" simply because downtime is a resource as vague as it gets... one gaming group might have LotR-style adventures where action is separated by months or years of idleness, while another group might have an endless series of adventuring days where all you get is long rests. Those activities
could have been balanced against each other, but to me it appears they preferred to just go with the narrative.
All said, the easiest and safest option IMO is just to grant the same boon to all PCs, and then wing the time to simply fit to whatever downtime you happen to have right now. Thus let every PC choose an extra feat and/or ASI, and count on the fact that there are feats to get various proficiencies (for those who aren't interested in a feat-feat). Make this happen when they do have reasonably long downtime and good facilities (e.g. they are going to stay at least a few months in a large city) and stress that there are favorable conditions (e.g. friendly trainers available), just to make it more believable, but don't worry about making it a formal rule.
Remember that if you make it a rule, then the players will believe that it is always
repeatable, and this might work against you.