Nothing huge has to happen to continue the series. As has been pointed out, they still haven't surpassed Eiji yet. Marriage or not, they haven't beaten their biggest rival.
Now Takagi is claiming they're going to make the final volume of Reversi outsell a volume of Zombie Gun and that will somehow make them number one in Japan and the superior mangaka.
Of course, they could simply get married and the story could just continue as they try to come up with something good within Takagi's specialty that can compete with Niizuma Eiji long-term.
There are still things to resolve with the side characters (Iwase ending Natural, redemption for Nakai, Hiramura - Aoki wedding, Nanamine round 3?) that could fill in as well.