In the same vein as my previous proposal, I suggest as small change for the old formats the replacement of current menu sprites (since the gen 6) by oldest (gen 3 to 5). This 2012 replay clearly shows that menu sprites was replaced at the arrival of XY (and they were motionless, unlike the cartridge). If we want to optimize all the way, we can even use menu sprites of gen 2 (see this battle in Crystal) and gen 1 (see this battle in Yellow). But it's more controversial to install because they are only 37 for 251 mons in gen 2 and only 11 for 151 mons in gen 1 (the risk of confusion between mons in the interface of the Showdown battles is important). To better visualize all that, I created here the list of mons by menu sprites (for compare between generations). And finally, here we have the image bank of these famous files.