Updated implementation of `BeginCollectionItem` to support nesting, not ignore the original `HtmlFieldPrefix`, and also provide overloads to let me decide the ID to use. Refactored to be a bit cleaner

CollectionEditingHtmlExtensions.cs

public static class CollectionEditingHtmlExtensions
{
    /// <summary>
    /// Begins a collection item by inserting a hidden field for the index value
    /// </summary>
    /// <param name="collectionName">The name of the collection property that owns this item</param>
    /// <param name="indexValue">The value of the index for the current item</param>
    public static IDisposable BeginCollectionItem<TModel>(this IHtmlHelper<TModel> html, string collectionName, Guid indexValue)
    {
        return BeginCollectionItem(html, collectionName, indexValue == Guid.Empty ? null : indexValue.ToString(), html.ViewContext.Writer);
    }
    public static IDisposable BeginCollectionItem<TModel>(this IHtmlHelper<TModel> html, string collectionName, string indexKey)
    {
        return BeginCollectionItem(html, collectionName, string.IsNullOrEmpty(indexKey) ? null : indexKey, html.ViewContext.Writer);
    }
    public static IDisposable BeginCollectionItem<TModel>(this IHtmlHelper<TModel> html, string collectionName, string indexValue, TextWriter writer)
    {
        if (collectionName == null)
            throw new ArgumentNullException(nameof(collectionName), "collectionName is null or empty.");
        if (string.IsNullOrEmpty(indexValue))
            throw new ArgumentException("indexValue is null or empty.", nameof(indexValue));

        //a hidden <input> element to hold the current index value passed in
        var indexField = new TagBuilder("input");
        indexField.MergeAttributes(new Dictionary<string, string>() {
            { "name", html.ViewData.TemplateInfo.GetFullHtmlFieldName(collectionName != string.Empty ? $"{collectionName}.Index" : "Index") },
            { "value", indexValue },
            { "type", "hidden" },
            { "autocomplete", "off" }
        });

        //Write the hidden <input> element
        indexField.WriteTo(writer, HtmlEncoder.Default);

        return new CollectionItemNamePrefixScope(html.ViewData.TemplateInfo, html.ViewData.TemplateInfo.GetFullHtmlFieldName($"{collectionName}[{indexValue}]"));
    }

    private class CollectionItemNamePrefixScope : IDisposable
    {
        private readonly TemplateInfo _templateInfo;
        private readonly string _previousPrefix;

        public CollectionItemNamePrefixScope(TemplateInfo templateInfo, string collectionItemName)
        {
            _templateInfo = templateInfo;
            _previousPrefix = templateInfo.HtmlFieldPrefix;
            templateInfo.HtmlFieldPrefix = collectionItemName;
        }

        public void Dispose()
        {
            _templateInfo.HtmlFieldPrefix = _previousPrefix;
        }
    }
}