In no particular order, all of the following would be welcome additions.

• Option to have a dynamic 'host extra clock time' that adjust deepening on the number of players in the simul. For example 3 minutes per player joined.
• Option to set a fixed start time for the simul. Arena's and other tournaments have a fixed starting time. It would be helpful to players to know when the simul will start, so that they are present and online when the simul begins and not otherwise occupied. This helps avoids 0/1 ply games (which also messes with the clock balance in a simul)
• A separate page for simuls you have participated in, either as a host or as a player. The page could be similar to the tournament page : https://lichess.org/@/Lance5500/tournaments/recent
• Upper and lower rating restrictions on players who can join. For example Lance5500 often have u2000 simuls. If such a rating restriction is used, we could remove the need for the simul host to accept players manually.

Currently the cap on simuls is 100 players. I think that Lichess should consider increasing that cap for titled players only.
While cool, I don't think we have enough request for this feature for it to be worth it. 100 is probably fine for now.

For reference here are some other similar issues relevant to general simul improvements that I would have included here, if there was not already an issues made for it.
#6410 Change simul settings (time) after creating.
#3888 Highlight simul host in chat