Well 14 is a capacity for 420 Pokemon, which is a lot. For a 721 hack I beliver there are roughly 391 Fully Evolved Pokemon so it would certainly fit there, but I don't about once gen 7 rolls in. Plus that doesnt consider a lot of legendary forms and duplicates etc.
It's not the most necessary thing every but doubling it to 28 boxes would be amazing for those 721 hacks. Fitting it in the Pokedex expansion could be good if you decide to do it. Maybe if possible have it like in Pokemon XY and ORAS where you only start off with the 14 boxes, but unlock the next ones when they each have a pokemon in it.