BigDebuffs, BattlegroundTargets and Omnibar for Classic?
 

Is it possible to make "BigDebuffs" and so on available for Beta a.k.a Classic? Or would that be too much work? I'm sure people would love that!

Or does anyone have anything similar to these addons that works on Beta?

Have you checked the respective addon pages to see if the author has said that they will look into making an addon for classic ? If not, I suggest you make a request post on that addon page ( or where the author has asked for such requests to go ) so that they are made aware of it ( if they track messages on their addon pages ).

Bear in mind that not everyone has got into beta yet so it may be a waiting game, after all it has only been a few days since they started giving access out.

If and when they do get into the beta it may be a lengthy process dependant on the functional differences the addon is affected with. Basically whatever functionality the addon currently codes for specifically that doesn't exist in Classic will have to be removed and similarly those missing need to be added.

Worse case scenario, you may have to wait for them to have access to Classic ( either beta or live ) or see if someone with access has managed to implement a version in the developers absence ( hopefully with their permission). I used to do this all the time with nUI when expansions came out and the addon needed fixing.