For example, if I add Full Combat Rebalance 2 mod to The Witcher 2, which drastically changes gameplay by adding new skills, weapon and armor stats, etc., is there any chance that my saved games could get corrupted/bugged and cause problems on The Witcher 3?

Yes, it might cause problems, but no normal customer can test it until it's released.

So I should hold myself from modding The Witcher 2 until The Witcher 3 comes out?

Nah, I would't, especially if you enjoy the mod, as it is really good. If the import eventually won't work, you're usually able to get savegames with your in-game decisions at the official Witcher forum (http://forums.cdprojektred.com), at the Witcher section of nexusmods (http://forums.nexusmods.com/index.php), here or elsewhere in the net. The import function was never an exact import with all your inventory, but rather a story driven import for continuity according to your former decisions. Thus it's not so important to use your personal savegame as long as the import contains the same decision structure. If I would have to decide between mods and import, I'd definitely choose mods, because you'll benefit much more from them than from the impact of an imported savegame.

Mods are more fun for the long-run, but what I truly care for are my decisions and personal savegames. I want to be able to import the savegames that I spent hours doing.
I know about these savegames on Nexus and RED-kit's forums. I don't use them because they only include the scarce major decisions, while ignoring all of the optional quests which, hopefully, TW3 will feature in a small way. (I'm gonna be very disappointed if CDPR ignores all of these decisions)

I guess my choice will be determined on what CDPR says about it. If they don't say a thing, then I will know until The Witcher 3 is released.

Honestly, I would guess that it would not be a problem. The way save importing from Witcher 1 into Witcher 2 worked, it only looked at the choices the player had made, and not at Geralt's skills. It did check his inverntory, but only for a few specific items which he could start with in the second game.

So I'm guessing that even if you were using FCR 2, when you import into Witcher 3 it will only care about the choices you made and maybe the names of some of the items you have. Changes to skills / stats probably won't affect it.

But again, no one can know for sure until the game comes out. Maybe try asking this question over at the CD Projekt RED forums? You might be able to get an official answer there.

I did, over a month ago. Nobody answered, so that's why I'm asking here.

If you haven't played the full TW2 game without FCR2 mod, then you should do that first anyway. Then you'll have an unmodded save you can use.

One thing I can say is this: players that used the FCR mod for TW1 didn't have any problems importing those saves into TW2. So I think the odds are slightly in favor of there being nothing to worry about.