I don't think space marines need to be buffed, other armies just need to be balanced.
everyone points at marines and is like "lol t4 3+ save fuck you guyssssss" yet most alien armies can outshoot marines at least 3/4 to 1.
ill use eldar as an example here, lets say 20 guardians get into a fire fight with 10 marines. lets say they both get to shoot at each other simultaneously to avoid complications on who is attacking etc.
10 marines fire 10 bolters, lets give them rapid fire too, so thats 20 bolter shots. with a bs of 4 that means on average 10-15 shots should hit, and 5-7 shots should wound.
eldar fire back with 40 catapult shots, bs 4 so 20-30 hit, 10-15 wound.
anyone who has played 40k can attest that its a numbers game, the player who can put the most dice on the table is going to have an advantage. now eldar would die without armour saves to bolter fire, so we can assume that 20 bolter shots on average would kill 5-7 guardians outright.
with the armour saves marines get 10-15 catapult wounds would kill 3-6 marines on average.
so with completely average rolls, 30-60% of the marine squad dies vs 25-35% of the guardian squad. after 2-3 rounds of this its not surprising to see that the marines have been almost wiped out.
Now this is just basic troop weapons, we know that alien armies field much heavier guns, marines do too but the alien armies have the numbers to quell that firepower, marines simply do not.
the problem with the game as it stands is marines don't have the capability to win battles of attrition. if you want to win as space marines you basically need to be a drop pod army or some cheesy ass speeder/bike army, where as with alien armies you can pretty much field whatever you want and have success.
Marines as they stand are a good list, the other armies just need to be tweaked.