The Halifax is too similar to the Lancaster for it to really be worth the large amount of work it takes HTC to add a heavy bomber. Performance is nearly identical, bomb load is only 500lbs less, guns are very similar. It had a higher loss rate than the Lancaster, so probably not as tough.
The only new RAF bomber we really need is the Wellington. I prefer the idea of the Wellington B.Mk III, but many suggest the Mk X.