We've found that Arkham Horror tends to have the opposite feel that it should - in the begining, there is a frantic rush to seal gates. Later on once you have a few gates sealed, you can move at a very relaxed pace. In fact, it often becomes apparent there is no way for Old One to appear at all, since there aren't really enough gate locations to build up enough Doom tokens, which also leads to the problem that its actually very BAD to close (without sealing) a gate, since it just lets more Doom tokens appear!

We tried changing things a bit, and really liked how this worked out.

When you draw a gate at a location with no gate, place a gate and a monster as usual, but DO NOT add a Doom token.

When you draw a gate at a location with an active gate, add a monster at the location and ADD a Doom token, but do not place extra monsters.

When you draw a gate at a sealed location, do not place a gate or doom token, but place one monster at every open gate (as you would have when a gate opened where there already was an existing gate in the normal rules). (This represents dark forces mobilizing in direct response to your investigator's efforts).

This seems to nicely alter the flow of the game. Early on, the Doom isn't building up, and you can actually afford to close gates. As more gates build up, the Doom track builds up faster. Seal gates, and its a big boon - but you will face more of a threat. And there's never a "safe" point where you know the Old One will never appear.

The other change we made, which is really quite seperate, is to reduce the number of monster strength trophies need to "cash in" to 3 (or 6 for those that would normally cost 10). One gate trophy would replace 3 strength. This lets you actually get to buy more stuff, and have more fun in choosing what to get and actually getting things like allies.

Edit: Whoops! Forgot to mention a minor thing. We did start out the doom track with one token on it.

Another Edit: Just for the record, credit for this variant goes a lot (probably more than me) to Nightmare.

A much, much later Edit:
We've kept tinkering around with variants; as the new expansion comes out, things change around. However, I don't have Dunwich, so any suggestion may fall apart with an extra board.

Last time we played we tried this:

At the start of the game, place a Doom token on the 0 space of the terror track. This is the Threat track. (It has nothing to do with the Terror level, that's just a convenient place for it).

Place one gate with one monster and one doom token to start the game as usual.

When a NEW gate opens:

1) Increase the Threat track by 1.
2) Roll a number of dice equal to the new Threat level.
3) For each die that scores a 5 or 6, place one monster on the gate that just openend.
4) If any of the dice scored a 1, add 1 Doom token to the doom track. Only add 1 Doom no matter how many 1's came up.

When a DUPLICATE gate opens:

1) Add 1 Doom token to the doom track.
2) A monster surge occurs.

This worked nicely for slowing down the start a little, but making duplicate gates even nastier and making new gates get progressively worse as the game went on (but not usually as bad as a duplicate gate). This also hopefully fixes the 'stalemate' problems some people reported where they just focused on closing gates; while closing a gate is actually a good idea in these rules, its still going to overwhelm you eventually.

The only question is what to do on sealed gates. Last play, we had sealed gates be safe, but that led to a lack of tension once we had 4 gates sealed. I'm thinking of trying:

When a SEALED gate is drawn:
1) Roll a number of dice equal to the Threat level.
2) For every two dice that scored a 5 or 6, place one monster at that location (even though it is sealed).
3) If at least two dice are scoring a 1, place a doom token on the doom track.

This would make Sealed gates much safer than open gates, but not totally risk free.

I'll give more comments when we get a chance to try it.