I actually have already done the last active thing by adding a notification dot on my board that shows the last active date/time when hovered over.
The code in my little plugin on my board:
{{if $comment->author()->isOnline()}}<i style='font-size: {setting="onlinestatusind_size"}px' class="fa fa-circle ipsOnlineStatus_online" data-ipsTooltip title="{lang="online_now" sprintf="$comment->author()->name"}, {lang="widget_member_last_active_date" htmlsprintf="\IPS\DateTime::ts( $comment->author()->last_activity )"}"></i>
{{else}}<i style='font-size: {setting="onlinestatusind_size"}px' class="fa fa-circle ipsOnlineStatus_offline" data-ipsTooltip title="{lang="widget_member_last_active_date" htmlsprintf="\IPS\DateTime::ts( $comment->author()->last_activity )"}"></i>
{{endif}}