I don't think RestSharp needs to introduce new method. Why don't you just make it work for client.Execute<dynamic> or client.Execute<object>
Here is how you would do for strongly typed currently.
var client = new RestClient("https://graph.facebook.com");
var result = client.Execute<User>(new RestRequest("4"));
var user = result.Data;
string name = user.name;
string id = user.id;
So make it dynamic if T == typeof(object) and response.content-type == "application/json",
In simple json if you pass <T> as System.Object or dynamic, it will just create either jsonobject/jsonarry/bool/long/string and other primitive types. So you can use dynamic.
string json = "{\"id\":\"4\",\"name\":\"Mark Zuckerberg\",\"first_name\":\"Mark\",\"last_name\":\"Zuckerberg\",\"link\":\"http:\\/\\/www.facebook.com\\/zuck\",\"username\":\"zuck\",\"gender\":\"male\",\"locale\":\"en_US\"}";
var stronglyTypeObject = SimpleJson.DeserializeObject<User>(json);
var stornglyTypedName = stronglyTypeObject.name;
dynamic dynamicObject = SimpleJson.DeserializeObject<dynamic>(json);
var dynamicObjectName = dynamicObject.name;
dynamic jsonObject = SimpleJson.DeserializeObject<JsonObject>(json);
var josnObjectName = jsonObject.name;
dynamic obj = SimpleJson.DeserializeObject<object>(json);
var objName = obj.name;
and .net 3.5/wp7 users where dynamic is not supported can still continue to use Execute<object> but instead of dynamic since it returns JsonObject/JsonArray so you will have to cast it to IDictionary<string,object> or IList<object>
var dict = SimpleJson.DeserializeObject<object>(json) as IDictionary<string, object>;
var dictName = obj["name"];
Here is some extra points on how we deal with FB C# SDK.
In the end this will look better for those that support dynamic.
var client = new RestClient("https://graph.facebook.com");
var result = client.Execute<dynamic>(new RestRequest("4"));
var user = result.Data;
string name = user.name;
string id = user.id;
And for those unlucky .net 3.5 and wp7 devs where dynamic is not supported you still partially benefit from it.
var client = new RestClient("https://graph.facebook.com");
var result = client.Execute<object>(new RestRequest("4"));
var user = (IDictionary<string, object>)result.Data;
var name = (string)user["name"];
var id = (string)user["user"];