I want to put the stats cap higher and lower the experience gain rate, and maybe not use training facilities altogether. Just for fun, cause I remember playing xcom apocalypse and having all my soldiers maxed out very early, then I felt it got less fun.
Well, it won't be easy.
Although what Reaver and Yankes said is all true, it doesn't help you much... because as I said at the beginning, you can't change how the stats improve, the algorithm is hardcoded.
a/ There's absolutely nothing you can do about TU, stamina, HP and strength stat improvement.
b/ There's almost nothing you can do about bravery, psi skill/strength and reactions... the new experience system can only help you increase them quicker... but you want slower
c/ melee accuracy is relevant only for mods... but if you're using some mods that depend on it, you can slow it down... a little
d/ throwing has been changed in OXCE and is now only trained when you actually hit an enemy with a grenade or something thrown... just throwing random stuff around doesn't do anything... so I guess here you can be happy at least
e/ as for firing accuracy, you can slow it down a little, same as for melee... but it probably won't be enough for you... even 33% experience gain compared to vanilla doesn't translate into 33% stat improvement... the formula for stat improvement is not linear... Last, most drastic thing you could do for firing accuracy, is turn off firing accuracy training for some weapons completely... but that somehow doesn't feel right.
My best advice... allow your soldiers to die... vanilla xcom is balanced in a way that even most veteran players can't keep them alive for more than 20 missions or so. Unless you of course don't use them; or use all possible cheesy tactics in the world. The game is MUCH more fun when soldiers die, believe me.