Tricky one Samantha,
Assuming that there are 365 days per year (usually), that each person who raises an Issue has an FTE of 1, and that it is the average per day within a particular year you are after (e.g. grouping your report by Entry Date, yearly), mathematically you can think of each Issue raised as being "worth" 1/365, from a SUM perspective (e.g. 365 Issues raised would be 365/365 = 1 per day, on average), in which case, you can effectively achieve the annual average you seek by doing a SUM using this textmode (in an Issue report view column):
aggregator.displayformat=HTML
aggregator.function=SUM
aggregator.namekey=view.relatedcolumn
aggregator.namekeyargkey.0=enteredBy
aggregator.namekeyargkey.1=fte
aggregator.valueexpression=({enteredBy}.{fte}/365)
aggregator.valueformat=HTML
displayname=Average Issue(s) per Day
linkedname=enteredBy
namekey=view.relatedcolumn
namekeyargkey.0=enteredBy
namekeyargkey.1=fte
querysort=enteredBy:fte
textmode=true
valueexpression=({enteredBy}.{fte}/365)
valueformat=HTML
Regards,
Doug