In need of office space? A new report from JLL details which downtown towers have the most available. Meanwhile, a slew of recently announced projects could add as much as 3.5 million square feet of it to areas in and around downtown Columbus in the next decade. The forthcoming arrival of all that new space – courtesy of high-profile projects like Scioto Peninsula, the North Market tower and an expansion of the Arena District – are raising the stakes for downtown's longstanding towers, especially…