I honestly wouldn't want that, a feature like multi-reddit would be much better IMO.

I personally don't want to be "automatically" subscribed to all tech communities for example just because I joined one, nor I want to be flood by an immense feed because all communities of the same type are put all together, that takes away individual choices IMO.

We had exactly the same problem on reddit, but multi-reddit solved that very well by leaving the choice to individuals instead of being forced by admins.

EDIT: for those who don't know, multi-reddit is a reddit feature that allows you to create different "labels" into which you can combine different subreddits, which label to create and which subs to combine is totally a user choice, those labels become "tabs" into your UI that you can use as they were individual subs.

So for example, I can create a label/tab called "linux" and use it to combine r/linux + r/linuxmx + r/xfce, etc., than I can create another label called "games" and combine r/MMORPG + r/wow + r/guildwars2, etc., and so on.

multi-reddits can be private, that is only the user who created them can see them, or they can be made public, so if some user doesn't want to create their own, they can use multis created by other people.