Edit: I've given it a lot more thought, and structured it a bit better.

Perceived Issues (short)

1. The current Era progression rules often lead to wide gaps between players
2. The incentives to switch Era are so strong that we can't stay for as long as we'd like in each Era
3. (2bis) Which also implies that unlocked technologies get obsolete too quickly, sometimes before we even had time to use them

Proposal (TLDR version)

In short, I propose that when selecting a Culture, players enter a configurable waiting pool instead of switching Era automatically at the next turn.

There would be some form of compensation for the players in the waiting pool.

Configure the pool at 0% to get the current behavior (no waiting pool).

Configure it at n% to wait for that number of players before the switch is triggered.

The aim of this idea is to keep the Era progression more consistent between players, to keep them "grouped"...

...although not completely, please read on for more details, and don't forget to vote if you like it :)

Details

About Issue 1 (era gaps)

In my opinion, wide Era gaps can be fine, but only if a minority of other players are left behind.

When it becomes a majority (as is often the case), it hurts the game balance and makes it far too easy for the most advanced ones (I often get bored by the end game).

About Issue 2 (strong era switch incentives)

There might be more, but I can think of the following incentives to race for the next Era

• Selection of the desired Culture

Perhaps the strongest, most obvious incentive

Note that two things happen here:

• we claim a new Culture
• we're scheduled to switch Era at the next turn

• Unlocking the next portion of the technology tree

Maybe you're maxed out, or you're shooting for a tech in the next Era

• Unlocking the next available wonders

Maybe all the wonders you're interested in are claimed

• Unlocking more Era stars

Maybe some groups of Era stars are maxed out, or all of them

Proposal

1) Waiting Pool

Decouple the Culture selection from the Era switch, and implement a waiting pool

Upon Culture selection:

• if the player is not in the current most advanced Era, then schedule the Era switch at the next turn (i.e. keep the current behavior)
• if the player is in the current most advanced Era, then add him to the waiting pool

The actual Era switch would still be done automatically but it would be delayed, until "enough" players (see 2) have entered the pool.

2) Waiting pool size

Add an option to configure the size of the waiting pool.

I imagine a simple slider controlling an integer percentage, where:

I believe 80% would be a good default value (or maybe 0 if you don't want to surprise the players).

This percentage can represent a number of things, discussed below. [my preference goes to 2.2]

2.1) a percentage of all the players

• the computed pool size would then mechanically be fixed for all Eras
• this would fix Issue 1, but a couple of "dying" players could halt the progression for everyone.

2.2) a percentage of the players in the most advanced Era, evaluated each turn

• if a player only just switched to the most advanced Era, this might increase the pool size, and we may (or may not) have to wait for one more player
• consequently, the computed pool size may become smaller and smaller as each Era goes by, but less so than in 2.3
• admittedly, it would not completely fix Issue 1, but it would limit it drastically and only a few players would be left behind (which is not a bad thing in my opinion)

2.3) a percentage of the players in the most advanced Era, evaluated once at the time the first player enters it

• we're not waiting for stragglers who may have just switched to the last Era
• it's more likely than in 2.2, that the computed pool become smaller and smaller as Eras goes by
• would still limit Issue 1, but less drastically than in 2.2

3) Waiting compensation bonuses

I believe it is important to maintain the current incentives to race for the next Era.

So when entering the waiting pool, players should start accumulating some form of bonuses, as a compensation for the wait.

Obviously, the first player to enter the pool will mechanically have earned more bonuses when the Era switch occurs.

Incentive 1 (culture race) would already be maintained in this proposal.

Incentive 2 (tech tree unlocking) would be delayed, so the bonus should be related to science : maybe just the player's research output accumulated during the wait ?

Incentive 3 (wonder unlocking) would be delayed, so the bonus should be related to that : maybe decreasing Influence and/or production cost ? Or the new wonders would be unlocked for the 2nd player in the pool only when the 1st has chosen one ?

Incentive 4 (era stars) would be delayed, so the bonus should be related to that : maybe just more Fame ? And of course, nothing prevents the player from working toward the other incomplete Era stars.