A daughter of Indiana and Marion, a grandson (via Mutt), plus Henry Sr. in Indy5?!
This is just speculation on my part and not based on any rumor.
On a side note Karen Allen turned 59 this Tuesday, October 5th. I think this has been discussed before but assuming her character is in Indy5 what is the minimum age she would be in the early 1960's? I would imagine Marion would have to been born in 1910 at the latest meaning at least 47 in KOTCS and in her early 50's in Indy5. I know YIJC showed Old Indy's daughter. If Indy and Marion got um "busy" right away in 1957 Marion could have had a child born in 1958 so a three or four year old daughter of Indy could be in Indy5. Its was rare but not unheard of for women to have children at that age in that time.
For that matter Mutt would now be old enough to have a child of his own, perhaps Henry Jones IV.
This may sound soap-operaish but it may be a mechanism by which Marion and Mutt have smaller roles in Indy5. Then again Mutt could live his kid with grandma Jones (Marion) who would babysit both little HJIV and his slightly older aunt (Indy and Marion's daughter). That way Mutt could join Indy on the adventure in some capacity.
Throw in Sean Connery as Henry Jones Sr's ghost and you have four generations of Joneses!