I see a few points that need to be addressed:

.Electing a sub creator What if several people want to create the same sub? 1/ First come, first serve? 2/ Kind of lottery? 3/ The one who pay more gets the sub? 4/ Community vote? I would tend to choose 1/ but not sure it's ideal.

. Conflicting names/themes What if someone want to create a sub on "Cryptos", another one on "Cryptocurrencies"? Probably better to let go, but could quickly be a mess.

. Sub-subs Shouldn't we try to organize subs by topics, rather than a flat structure will all subs at same level? (for instance /sport, then /sport/football, sport/tennis...)

. Subs limit per person What if someone decide to create dozens of subs? Is it allowed? To avoid excessive noise, probably better to put a limit (say, 3 subs max per person).