In Unity 2019.1 Unity added a new custom EditorTool API, heres some example code showing some tools we could make with it!

LookatTool.cs

using UnityEditor;
using UnityEditor.EditorTools;
using UnityEngine;

[EditorTool("LookAt Tool")]
public class LookatTool : EditorTool
{
    GUIContent cachedIcon;

    // NOTE: as were caching this, unity will serialize it between compiles! so if we want to test out new looks,
    // just return the new GUIContent and bypass the cache until were happy with the icon...
    public override GUIContent toolbarIcon
    {
        get
        {
            if (cachedIcon == null)
                cachedIcon = EditorGUIUtility.IconContent("ViewToolOrbit", "|LookAt Tool");
            return cachedIcon; 
        }
    }
    
    public override void OnToolGUI(EditorWindow window)
    {
        var view = SceneView.lastActiveSceneView;
        
        // If there are multiple selected objects, we want to focus the look position somewhere in the middle..
        if (Selection.transforms.Length > 1)
        {
            Vector3 center = Vector3.zero;
            foreach (var transform in Selection.transforms)
            {
                center += transform.position;
            }

            center = center / Selection.transforms.Length;
            view.LookAt(center);     

            // Lets draw a tiny sphere at that center point, so we can verify it works
            Handles.SphereHandleCap(0,center,Quaternion.identity, 0.1f, EventType.Repaint);
        }
        // If we only have one selected object, thats way easier, we just look at it
        else if (Selection.activeTransform != null)
        {
            view.LookAt(Selection.activeTransform.position); 
        }
    }
}

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Always thankful for your support, Lotte💖