Anyways, any of the suggested changes would pretty much cripple OOS due to the small selection of viable cards. Maybe people want the Champion mechanic gone?
I think we all remember what the Overseers were like before they got Champions: play all of the cards together mindlessly, profit. At least now the deckbuilding requires skill even if the playstyle is semi-mindless flying rush.
Now, two sets ago you could easily say that the Overseers didn't have enough cards and nerfing their key ones would be too disastrous for the faction as a whole, but I dare say the Overseers have grown up and can't hide behind "I'm a new faction! Don't hurt me! I need more cards first!" The "small selection of cards" argument was one of the issues when the Champion mechanic came out (people hated it, mostly because there was no real benefit to running multiple champions aka not Sol), and has since resolved itself. So I don't think that's the real danger here for them.
Can you nerf the Forerunner of Champions or Champion's Companion and still have the Overseers survive? Probably. There would be doomsayers, of course (just like with Demon of Fear), but the faction would survive.
Forerunner of Champions can probably get the Aether Acolyte treatment: increase base stats, reduce buff (4/4 getting +2/+2 every time, for instance), or something similar (2/2 getting +2/+2 every time, perhaps)
Champion's Companion is tricky, because it's the mechanic that is so strong and not the stats. Given that you could nerf the Companion's stats into oblivion, but that may not have an actual effect on the real problem of just swapping out Champions to prevent them from dying to targeted removal. You could have the swapped out Champ remain on the field as a shadow-card (the visual you get during the planning phase anyway) that disappears after the ability phase--this shadowcard would be vulnerable to removal and would also destroy the version that had just been bounced to your hand. I actually don't think that'll work just from a programming perspective, but I'm not sure how to make Champion's Companion weaker without just nerfing all of the individual champions to assume that he's in play. Because that's no fun. Perhaps you should do as someone earlier suggested and make him a 4-drop, because that interferes with the curve (Lucca, Lilariah, Harahel, etc; 4-drops have a very competitive spot on the resource curve)
And maybe the Overseers don't need a nerf at all and this is just people complaining in what TheRealTuna has described as a "pretty healthy" meta. I don't know. Balance is complex, and boy is balancing things hard especially with so many moving pieces. Here is a fact, though:
If the "key" Overseers are nerfed or changed in any way, no matter how small, you will have people saying that they're complete garbage and won't see play anymore and gg (and no doubt a few will threaten to leave the game). And then life will go on and someone may come up with a new combo for the Overseers that is just as powerful as the current one but was getting crowded out by Forerunner/Tempus. Then people will complain about balancing, there will be forum posts much like this one, and the cycle of life and death will continue to crescendo-ing music as the screen zooms out and fades to black...