There really aren't any good mods out there that fix what is, in my opinion, a sadly lacking part of the game. Despite being incredibly expensive, beacons seem to be... not all that useful, and even kind of anticlimactic of an item, seeing as you have to both kill the wither and gather an enormous amount of resources just to make one. Stranger still, while they can be made out of any mineral block, rarer, more expensive minerals do not provide so much as a single benefit. The process of supplying them with a fuel input isn't necessarily bad, but seems completely useless considering the ridiculous amount of resources you have to gather just to build a beacon in the first place.
There are plenty of improvements that could be made to beacons- namely, each type of beacon could have a different proficiency that would make it worthwhile to gather more minerals, or even to make a beacon out of multiple mineral block types. Perhaps Iron is basic functions such as mining boost and efficiency increase, while emerald provides fertility and regenerative properties, diamond provides powerful defensive properties, and Netherite offers powerful offensive properties. A beacon made entirely out of one material would only offer the corresponding properties, whereas a beacon made out of multiple materials could offer lesser versions of each.
Iron ~ Standard boosts such as Haste and Mining Increase, that sort of thing. Iron is the most basic sort of beacon, and its abilities should reflect this.
Emerald ~ Provides some special, more niche benefits- this could include saturation, a crop growth or yield boost, health regeneration, and perhaps a significant increase in livestock offspring, tree growth speed, that sort of thing.
Diamond ~ Defensive properties of a powerful nature, such as an energy shield that keeps out hostiles and prevents them from spawning inside- possibly by providing a powerful ambient light source. Perhaps the diamond beacon can charge the player in a manner similar to a charged creeper, giving them a powerful defense boost and some sort of thorns property. Perhaps it reigns lightning on hostile mobs?
Netherite ~ Whatever the nether beacon does, it should be incredibly overpowered, and possibly very cool. I'm thinking lasers of some sort. Yes, definitely lasers.
Other Notes ~ Beacons can be a bit underwhelming considering how hard they are to make. The noises are cool, and the actual beacon looks cool... but maybe some runes circling around the beam? Some sort of cool particle effects? Maybe blocks connected to the beacon pulse with strange markings?