Heres what im saying what if the Moon Lord was not the final boss what if there was a worse enemie that the Moon Lord and if it is what would it be called?
The issue with a post Moon Lord boss is cyclical - where do you possibly go from here in terms of boss attacks and rewards?
If you were to make a post Moon Lord boss, it either has to be balanced around Celestial gear and get basically curbstomped by equipment like SDMG and Last Prism, or it has to be balanced around that equipment and have even more health and damage to compensate for the sheer, unadulterated power that players have to their names at that point.
Additionally, what would the reward be? If an even more difficult boss is added, it either needs to drop sufficiently powerful items and gear as a reward, or it simply drops nothing but maybe a trophy to keep the gear from getting any wilder. If you opt for the former, you'll have a similar issue with Moon Lord as it is where people will say "there's nothing to use this new gear on!" and if you opt for the latter then you'll get numerous complaints that the boss doesn't feel rewarding enough to battle.
Another funny side effect of adding even more gear would be that you'll end up with similar discussions assuming there's new gear. Like, say we actually do get a boss named Sun Lord which you can battle after defeating the Moon Lord.. if it drops new gear, then people will be asking for a post Sun Lord boss instead.
If I had to make a suggestion for post Moon Lord content though, mine would just be to have Duke Fishron get stronger if you've cleared Pillars or Moon Lord beforehand in order to deal with the gear tier progression.. but even that isn't a very elegant solution as it removes an available option for players who are less adept at battling Duke Fishron to just overwhelm it with raw power, and I am against removing options in a game like Terraria.
We also have no idea what the 1.3.6 power curve is going to look like either, so that may change player opinions depending on what happens. I'm personally looking forward to it.