Most likely you have already come across the SAP Fiori applications and might therefore be wondering what architectural options there are to implement similar easy to use applications on top of your SAP backend systems leveraging SAPUI5 on top of SAP Gateway.
The potentially least complex option for SAP Fiori style applications that only require connection to one backend system, e.g. SAP ERP, is to implement all SAP Gateway and SAPUI5 components onto that backend system.
Once you require connections to more than one backend system, e.g. to SAP SRM in addition to SAP ERP, an OData Hub becomes the more viable option. In this scenario it would make sense to leave the SAPUI5 frontend components on the OData Hub as long as you consume the solution locally in your corporate network and don’t require access through SAP EP.
If you prefer to access your SAP Fiori style applications via SAP EP, e.g. to simplify SSO, you should consider deploying the SAPUI5 components to EP and thereby form an UI Hub. However SAP does currently not support this configuration for their native SAP Fiori apps.
In case you would like to access further PO components like BPM or BRM it would make your solution more flexible and reusable if you accessed those components through the OData Hub as well rather than accessing them directly from SAPUI5, e.g. by combining them with SAP EP.
To open your SAP Fiori style applications to users outside your corporate network you could of course connect them via VPN or open up your internal network to the outside world, preferably through a DMZ, but it might be easier and architecturally more flexible to make use of the SAP Hana Cloud Platform. In this scenario you would deploy the SAPUI5 components onto the SAP Hana Cloud and use the SAP Mobile Platform, enterprise edition, cloud version in case of mobile users. Via the SAP HANA Cloud Connector you would securely connect to your OData Hub. Unfortunately SAP does currently not support this configuration for their native SAP Fiori apps.
There are more architectural options, especially in detail, for your SAP Fiori style applications but the above selection should give you a good starting point to evaluate your respective strategy. Therefore I would like to mention only one more option and that is to move one or more of your core SAP backend systems to an on demand provider, e.g. SAP HANA Enterprise Cloud. In this case you would securely connect to your OData Hub via the SAP HANA Cloud Connector again.