When mode isn't a subcategory it's basically treated as an unimportant variable. Here different modes actually mean completely different categories so it should be a subcategory.

I agree with this

I also agree with this, and subcategory names should be typed without spaces (osu!, osu!taiko, osu!catch, osu!mania).

agree with YaLTeR

I have to agree with Awsumi, it would just end up looking very messy, you can click filter > mode > osu!standard and see all the standard plays if you wish. If you have a speedrun that's for example 3:33 on standard but 3:30 on multiple modes, the 3:30 will show up on the global leaderboards, and the 3:33 will show up on the filtered to standard leaderboards, so nothing overwrites :D