Repotting is not just the time to remove excess roots and refresh the soil. That’s a huge part of it but repotting also gives the bonsai artist an opportunity to dig around the nebari and see if the tree has developed more interesting roots that can be revealed by lowering the soil level. I have found that the jet spray on a trigger nozzle can effectively remove the top of the soil to better reveal the roots without damaging them by scraping away. Moist soil or moss on surface roots can hasten the thickening of them. If nothing interesting has developed, cover them again and hope for a better outcome next repot.
Has the back of the tree become the front? Can you change the orientation to better show off growth and development improvements in the tree? Can the angle of potting be altered to improve the shape of the tree or bring an entire new look to the bonsai? Is there a better position for it in the pot? A bonsai is never static and close observation of the changes at repotting time is essential to get the best from your tree.
Do you have a different pot that will bring out the character of the tree better than the one it has been in? It is fun to change pots each repot. Anew pot gives a tree a fresh look and brings more interest. Some trees stay in the same pot for years because it suits them but others can benefit from a pot that is different in colour, shape or texture. Is it time to put the tree in a smaller or larger size pot? Is the shape of your pot right for the tree? If the shape of your bonsai was even last year and it is in a round pot, if one side has extended then possibly an oval pot is better for it.
And while on the subject of repotting… When you get a new tree, never start to style it without removing the soil surface to find out where the nebari starts, how the trunk looks that has been revealed by removing soil and assessing any faults or benefits. Then you can better determine the angle of potting and the styling of the tree.
Repotting is a fun time while you work to show your tree to the best advantage. All of these are important issues when repotting, not just removing excess roots. The more attention that is paid to them, the better the tree.