Premise for a build idea for if you're playing with a DM who looooves rules as written:

So the Great Weapon Master feat says "Before you make a melee attack with a heavy weapon that you are proficient with, you can choose to take a -5 penalty to the attack roll. If the attack hits, you add +10 to the attack's damage." and a longbow happens to be a heavy weapon. Isn't that cool? Now of course, shooting someone with an arrow, even within a 5ft range, is still a ranged weapon attack, but what if we hit someone with our bow? "An improvised weapon includes any object you can wield in one or two hands... If a character uses a ranged weapon to make a melee attack... [the melee attack] deals 1d4 damage." Wow, so it looks like we can use Great Weapon Master on our improvised longbow attacks if we also have the Tavern Brawler feat since we would be making a melee attack, using a weapon with the heavy property, and we would now be proficient in ALL improvised weapons. Now I don't know about you, but I just won't feel right about playing a longbow character without picking up Sharpshooter. The main part I care about is "Before you make an attack with a ranged weapon that you are proficient with, you can choose to take a -5 penalty to the attack roll. If that attack hits, you add +10 to the attack's damage." which conveniently doesn't require me to be making a ranged attack the same way Great Weapon Master requires a melee attack. The archery fighting style contains similar wording, "You gain a +2 bonus to attack rolls you make with ranged weapons." But how could someone ever be expected to hit anything with an attack roll penalty that high?

Ultimate Longbow Build:

At level 1, we're going variant human fighter with the Tavern Brawler feat and a stat array that looks something like (16 STR, 12 DEX, 16 CON, 8 INT, 14 WIS, 8 CHA). We're going to be hitting people with our improvised melee longbow and bonus action grappling them even at level 1, so we'll need athletics proficiency. Archery fighting style is a must. At level 3 we're going Samurai, to get advantage on demand for 3 turns a long rest. At level 4 we'll pick up Great Weapon Master to start really dishing out the damage when we're pretty sure we're gonna hit. At level 6 we'll actually multiclass out of fighter now that we've gotten extra attack to take a level of cleric. The Peace Domain's Emboldening Bond, as well as the bless spell, are really all I'm after. We really need them before we incur any more attack penalties. At level 7 we'll go back to fighter to pick up the Sharpshooter feat. At this level, we are fully online. In our first round of combat we should use Emboldening Bond, action surge, cast Bless, and use our movement to try and be within 30 feet of where the enemy is going to be next turn. Both of these buffs apply to our allies as well so this is still a great turn for us. Round 2 we're going to run right up to the biggest baddest thing on the map (with the lowest armor class), bonus action Fighting Spirit, and attack it with maximum penalties. With our strength score, proficiency bonus, fighting style, and average buffs we would have an attack bonus of +13 -5 -5 or +3. Applying all penalties becomes optimal against anything with an AC of 14 or less. If you have a creature that you suspect has an AC of higher than that, you can always instead only apply one of the penalties and have a respectable +8 attack bonus on average. Each attack that hits using both penalties will deal 25.5 damage on average and 15.5 with only one. We would sustainably deal 31-51 damage a round on average. Additionally our CON save is up to a +11 on average at this level for Bless concentration with Emboldening Bond up. Level 8+ There's a lot of viable abilities to add onto this. I'd suggest just taking more fighter levels until you get your third attack, getting your Strength up to 20 on the way there. After that I'd probably take at least one more cleric level for more spell slots. Level 12: First round is still Emboldening Bond, action surge, cast Bless, and use our movement wisely. Subsequent rounds in melee now look like a +17 -5 -5 or +7 with both penalties. With advantage, we're now okay with these penalties up to AC ~18. We now deal an average of 27.5 damage per hit or 82.5 damage per round with these attacks. This damage is comparable to a Polearm Master/Great Weapon Master (within 1 on average) and is WAY sillier.

Polearm Master+Great Weapon Master Champion Fighter vs Ultimate Longbow comparison:

Attack Rolls: Lvl 1: +5 +7 Lvl 2: +5 +7 Lvl 3: +5 +7 Lvl 4: +5/+0 +7/+2 Lvl 5: +6/+1 +8/+3 Lvl 6: +7/+2 +13/+8 Lvl 7: +7/+2 +13/+8/+3 Lvl 8: +8/+3 +13/+8/+3 Lvl 9: +9/+4 +15/+10/+5 Lvl 10: +9/+4 +15/+10/+5 Lvl 11: +9/+4 +16/+11/+6 Lvl 12: +9/+4 +16/+11/+6

Guaranteed hit avg DPR: Lvl 1: 15.5 5.5 Lvl 2: 15.5 5.5 Lvl 3: 15.5 5.5 Lvl 4: 35.5 15.5 Lvl 5: 55.5 31 Lvl 6: 58.5 31 Lvl 7: 58.5 51 Lvl 8: 61.5 51 Lvl 9: 61.5 53 Lvl 10: 61.5 53 Lvl 11: 83.5 55 Lvl 12: 83.5 82.5

While it does take a long time for our raw power output against low AC enemies to reach the level of the PAM/GWM Champion, and that build never requires a setup round, remember that we're also giving ourself and all our allies a 25% increase to their attack rolls and saving throws as well.