Until today I never use web services on sap and want to switch to them. My .NET application is still using Sap Connector.
For the future, I think it will be good, when I would build an interface based on webservice or rest api for access sap data.
My question, which opportunity do I have to build web interface or restapi on my sap system?
Can I use SAP UI5?
The simplest option you have is to create a Remote enabled Function Module and then create a web service from it. Your .NET application can use this web service (so that you can have SAP data).
SAP UI5 is altogether different. It is used to build SAP applications and not to expose data outside (in this case to your .NET application).
If you have SAP Gateway installed (component GW_CORE, level 2) you can use OData.
If it's not installed, check with your Basis team if, for your system, it's possible to add the component.