Draft of recentering while running isearch.

isearch-recenter

(defvar my-isearch-window-start '(nil 0)
"Record the `selected-window' and the Y-axis coordinate of point each time that
`isearch-update' calls `my-isearch-update-recenter' (via an advice).  The value
of this variable will be a cons cell with the CAR being the `selected-window' and
the CDR being the Y-axis coordinate of point.")

(defun my-isearch-update-recenter ()
  (let* ((win (selected-window))
         (pt (point))
         (y (cdr (nth 2 (posn-at-point pt win))))
         (same-y-p (and y
                        (eq win (car my-isearch-window-start))
                        (= y (cdr my-isearch-window-start)))))
    (when (and y (not same-y-p))
      (recenter)
      (setq my-isearch-window-start (cons win y)))))

(advice-add 'isearch-update :before 'my-isearch-update-recenter)