he options aren't great, from the right I'd expect Anas Sarwar will likely run again and probably be the favourite, a few years ago you'd have expected the left candidate to have been Neil Findlay (but he's retiring) or former deputy leader Alex Rowley (who has basically receded from the spotlight after a sex scandal), now I'd expect it might be perennial candidate and Glasgow Councillor Matt Kerr.
I believe the rules say that the leadership candidates must be MSPs or MPs (or MEPs but this doesn't apply anymore). This would exclude Kerr.
The timeble is the following
Sunday 17 January, midnight
Deadline for declarations of interest
Monday 18 January
Nominations open for Leader
Tuesday 19 January, noon
Nominations close
Validly nominated candidates for Leadership to be published
Scottish Labour hustings period opens
Supporting nominations open
Wednesday 20 January, 5pm
Last date for members to pay off arrears in order to participate
Tuesday 26 January
Supporting nominations close
Tuesday 9 February
Ballot opens
Scottish Party hustings period closes
Friday 26 February
Ballot closes
Saturday 27 February
Result announcement
Meanwhile, the selection ballot for list ranking is also taking place with voting closing on January 29th. Given the state of SLAB, almost (if not all of them) all seats will come from list section. Therefore, the ranking is crucial. However, they didn't seem to have attracted many applications.
Leader and Deputy Leader are guaranteed the top spot in their regions. So Leonard didn't enter the contest. However, now he's not the leader anymore. It seems they agreed to keep him at the top of Central Scotland list.
I suppose stopping and re-run the ballot to allow him to join would have been just a waste of time.