Could I change the fields of description in WYSIWYG?
Which field do you mean exactly? The item descriptions?
Yes 
Is it possible?
No, it was remove for now because of security issues. But it may be included again if there is a proper way to allow formatting like bold, italic, whatever in a security-save way. The way to go would be using Markdown.
So is there currently a way to make a simple list in the description field of an invoice?
No,because any htmlentities are stripped before render.
Yes, but I can easily switch that of in the invoice template.
<?php echo nl2br($item->item_description); ?>instead of
<?php echo nl2br(htmlsc($item->item_description)); ?>