Use case for this in core is the "Number of items to index during cron runs" setting in admin/config/search/settings:

It currently allows for a choice between 6 pre-defined values, as shown in the screenshot above. Ideally, we could allow any custom value (grater than 0), and offer these values as suggestions, instead of having them hard-coded.
The HTML5 number field allows that as follows:
<input id="ticketNum" type="number" name="ticketNum" list="defaultNumbers">
<span class="validity"></span>
<datalist id="defaultNumbers">
<option value="10045678">
<option value="103421">
<option value="11111111">
<option value="12345678">
<option value="12999922">
</datalist>
Which makes the <input> field a free-type number field + offers the suggested values in a drop-down in the same field:

Recent comments
Thank you, this is a very useful workaround for preparing a clean import list. One concern remains, however: if Simplenews 1.x-1.2.3 still treats blocked user accounts as invalid...
Re-enable Simplenews newsletter subscriptions that had been erroneously disabled
@Amilenkov, I have an idea for your use case where you want to avoid re-subscribing people who have explicitly opted out: Build a view of simplenews subscriptions, and filter by Status (...
Re-enable Simplenews newsletter subscriptions that had been erroneously disabled
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