My mum would be a better opponent than that fraud Goldberg. The guy is not a wrestler - he's just a football player who does other things here and there. Holy crap, Hogan would be a better opponent for Undertaker, at least the guy is dedicated to the 'sport'.
Jericho is a good suggestion and he has no problem putting people over. But if you consider that we won't be seeing The Deadman until next year, then finishing off his feud with Kane will be priority. Given the small window of time to end that and start towards something else and make it credible and worthwhile, that mean 'Taker would have to beat Kane in the first 2 weeks of January, giving us a really cheap climax. IMO, the best buildup for WM next year would see Kane continuing to dominate Undertaker until Wrestlemania and have 'Taker go over big time there to end it.