By big fish, do you mean like a centerpiece or just fish bigger than, let's say, about 3-4" in general?
Anyways, here are some fish to consider:
Zebra Loach - not really a centerpiece fish, best to have a group
German Blue Ram - you could stick two in a 29g
RTBS - you might be able to just have one of these guys, i dunno; imo, i think theyre too large for 29g
Firemouth(goes together with Convict)
Convict(goes together with Firemouth)
Those are some fish I was considering for my 29g. maybe a group of zebra loaches and 2 german rams? or 1 firemouth and 1 convict. That's what I would do if I restarted my 29g. I'm pretty sure if you get an rtbs, you'll have to keep him alone.