I know that up to now all available companions can only be obtained by doing a specific quest in one of the new dlc zones.

This is a bit sad for those who don't own one of these zones.

I understand the marketing reason to hide companions in dlc zones but at the same time I am quite certain that people won't buy a dlc just to get access to a companion. People will buy a dlc because they want to play the new zone, get access to specific houses or do the new story and trial and the companion is just an addon.

I think players would more in general appreciate to be able to get a companion even if not owning the respective dlcs.

Also, there are players that own these specific dlcs but running around with a toon and doing zones without a companion and would like to do the zones with a companion not at all or later and would not like to hop to that zone with a companion just to get a companion to run along.

So here is my suggestion:
On a random basis a quest could pop up in one of the base game zone cities or non-companion dlc zone cities that give access to a companion not yet inside the game. The associated quest could be placed for instance in any open dungeon requesting to kill a number of bosses there or a number of zone word bosses or do a number of normal dungeons or whatever. Something that is pretty generic and I am certain the story writers at ZOS could come up with a nice idea to embed it in a story (e.g. related to the Undaunted or so). Thus people could have with a lil bit of luck get access to a companion without the need to interrupt their playthru with a given character.