Yes I am hoping for a Matherine baby. I agree maybe Katherine might be able to heal the heartbreak that Elena gave him by breaking up with him pre-Season 1. Plus it might be able to heal Katherine's heart also for having to give up her baby. It would totally work. <3 Matherine.
Connor from Angel worked particularly well. He was one of my favourite characters, so if written properly they can work well. I have high hopes for Klaus and Hayley's baby I hope it grows up supernaturally quick so it can be a major player in the scheme of things and Klaus was all for Elena having a baby with Matt whilst she was still Human (see episode 3x13) so now that Katherine is Human he would want the same thing for her, to carry on the Petrova line so he can make more Hybrids in the future. Katherine having another child is inevitable IMO.