There is a long term 3.2 RNAV trial going on, and there have been trials of steeper segmented glide slopes (4.0 degrees then changing to 3.0 degrees at 4DME, that type of thing) as well.
There are a few problems.
Once you get above 3.2 then CAT III approaches are not going to be flown (that’s the certification limit for most airliners (well, 3.25)).
Once you get above 3.5 degrees then energy and speed management become very critical, so the chances of aircraft being able to maintain an ATC-specified speed down final approach with minimum separation both ahead and behind become slim to non-existent. On some of the vertical segmented trials, 777s had to be flying at 160kts at 25nm from touchdown joining the 4.0 degree slope to ensure they were still flying 160kts at 4DME and able to slow to finally approach speed. Modern airliners are so efficient in terms of drag that they are difficult enough to slow down without hanging everything down which actually makes them a lot more noisy than a cleaner aircraft on a 3 degree slope.