In theory Honda could enter the Accord since it's about the right size, has a sedan version and has been built in Ohio ever since the line started. And since NASCAR uses fuel injected engines now, their one big reason for not coming in when Toyota did is gone.
I don't really see it happening though, because Toyota had successful grassroots support which spurred the decision to move into the upper leagues (including the guy running a Celica in the now-defunct Goody's Dash series that got a lot of attention at the time), and like you said I can't see any new manufacturer spending the huge amount of money to get established unless they can hook at least one big team right off the bat.