If you look at the DLCs of DS3, you can see that for almost every boss, there is a lead up for it. Most notably, Midir follows you throughout the Ringed City, watching you, then attacks in an ambush trying to finish you off once and for all. The greatwolf attacks you in two different spots, Freide (politely) asks you to leave, and Gael helps you through messages and summons throughout the DLC.
This to me is how the base game should have added more chances to reveal the bosses before their arenas, as the only real pre-reveals are the intro cutscene and the statues of NK leading up to the Wyvern.
Here's what I would add:
After killing the Deacons, Aldrich bursts out of the Coffin in play-doh form, and destroys the roof while flying to Ithryll. During the cutscene Anri and Horace are firing arrows at him to no success then collapse after the cutscene ends.
In the Farron Swamp, there are ambushes of NPC watchdogs with greatbows in trees and single Abyss Watchers in the fat roll poison areas. None of them walk nor fat roll through it though. These should be really hard for any soul level to beat solo. The goal would be to have to avoid these traps that you can see from a distance if you look carefully as you run to their lair.
Throughout the profaned capital, small doorways and walls should have been destroyed to show show the path of destruction Yhorm had to make in order to get back to his throne.
You can see Dragonslayer armor being carried by a butterfly in the distance even from the High Wall.
If done correctly, my hope would be that after seeing these, you would think "Wow, I want to fight this thing!"