We will be doing a new Intranet for our SharePoint Online. I have experience developing in SharePoint 2010 / 2013 but using Farm Solutions. In SharePoint Online, I believe the only options are App and Sandbox Solution, but I read that Sandbox solutions will soon be deprecated.
Or if Sandbox solution is still preferred for this level of project, how do we start developing since we don't have SharePoint On-Premise? Is it possible to just copy the ISAPI folder and required dlls and develop on our local machine?