A lot of app developers like the idea of adding a chat function to their app. In UI for WP Telerik has this beautiful Conversation View. Make the available as a full-blown widget für Kendo UI (mobile). Think of SignalR as a possible backend technology. Chat bubbles should have the ability to show text/pictures/emoticons. Provide a ready2use class for asynchronous or backgroud-sync (up/download) e.g. for ASP.NET Web API.