Wow - I just discovered this 'hack' (appropriate for the aim of the game!) and somehow I find myself near the top of the Leaderboard! So I thought I'd better make a few comments...

Great game - it's kept me from work through the day which has been a welcome relief...

Some comments on the various aspects of the game:
It would be great to see some more regional flavour to the game - I'm not sure how the monsters are coded, but it would add some great detail to face monsters that were more suited to the area. I see that's been put into place for the City Bosses, but it'd be cool to see regional monsters as well.
For example: instead of Tigers everywhere, maybe they could be restricted to places like New Delhi and Beijing, while you'd see Lions in Harare and Cougars in Denver and Calgary. Bears could be differentiated into Grizzly, Polar, Brown, and of course Panda. Ogres could be replaced with whatever mythological beast roams that land, like Ghuls or Djinni for Riyadh, Mummies for Cairo, and Hillbillies for Atlanta. Snakes could be easy to differentiate into Pythons, Cobras, Rattlesnakes and Tiger Snakes. Gators are easy to break down into alligators, crocodiles, goannas, and such. And who wouldn't want the chance to face killer Kangaroos, Koalas and Wombats in the wilds near Sydney?

I had an idea about some more items for the game, as separate from Potions. Different city stores or secret locations could each have a different special item, and maybe each character could only have two or something like that to keep things under control? If you added these I'd suggest you break Potions off into their own category.
Some ideas I had are listed, and many could be scaled into minor or major or related versions. I haven't given thought to costs but there are definitely some here that seem more 'Level 1', while others are clearly 'Level 4'. Some you may consider a bit broken but I thought I'd get all the ideas I had down.
  • Boots of Speed Allow the wearer a better chance to strike first in each combat round
  • Belt of Strength Add a bonus to the bearer's combat skill
  • Helm of Defence Reduce a percentage of monster attacks
  • Crown of Defiance Add a bonus to bearer's damage vs City Bosses
  • Necklace of Negotiation Reduce the cost of item purchases
  • Scroll of Fire or Ice Enable a Warrior to cast single-use spells
  • Gem of Telekinesis Enable a Wizard to deliver Warrior-class physical damage for a single attack
  • Bow of Swiftness Automatic first strike
  • Vial of Frost Render enemy counterattack useless for a single or multiple rounds
  • Astra of Slaying Destroys a single (non-boss) monster instantly!
  • Shastra of Shielding Maximum damage attack reduces foe's next attack by half
  • Spear of Piercing Maximum damage attacks deal double damage
  • Dagger of Venom Instantly reduces enemy (non-boss) health by half
  • Pendant of Accuracy Instantly reduces City Boss health by a third
  • Gloves of Ambidexterity Enable 2 attacks versus enemies single attack per round
  • Hat of Summoning Enable trading at shops whilst still in combat
  • Glasses of Assistance Attacks dealing less than average damage deal average damage instead
  • Stone of Potency Single-use item causes next spell to deal double damage
  • Ring of Energy Reduce mana cost for spellcasting
  • Dice of Daring Automatic maximum damage while characters' health is below a third of maximum
  • Clover of Luck Reduce mana cost for all spells while bearer's health is below one third of maximum

It would be good to see a Japanese city and perhaps one situated on a Pacific Island or New Guinea? Antarctica would be another great location albeit without as much to look at! And some ideas for the bosses that appeared unnamed as at the time I played include:
Buenos Aires: Giant Anteater, Great Andean Condor, Yarará Viper
Harare: Shona Warlord, Great Zimbabwe Bird, Giant Bateleur Eagle
Madrid: Toro Gigante (Gigantic Bull), Fascist Strongman
Mexico City: This had a Golden Eagle, but maybe a Feathered Serpent would have better resonance.
Moscow: Mafia Boss, Cold War Cyborg, Red Army General
New Delhi: Rakshasa, Nether Asura
Paris: Giant Rat, Walking Guillotine
Riyadh: Great Efreeti, Djinni Lord
Taipei: Tigress Witch, Rebel Dragon

Other stuff
Some non-city locations I thought of could be stuff like this:
Halls of Chance: Spend a small amount of cash (perhaps a half or third of the equivalent potion) to gamble for the chance of a healing effect. There could be some Halls for healing, others for mana, and rare locations to gamble for both (ie a Life Potion effect).
Lairs: These locations could hold monsters more powerful than the regular wanderers, but less than City Bosses. Maybe they would garner more XP or Gold, or perhaps magic items upon defeat.
Strongholds: Maybe higher-level players could create their own keeps to store items or gold, in case they lose in battle and don't want to lose the gold to the monster? There would be a chance of robbery from other players or wandering monsters if the player doesn't visit regularly, or perhaps pay a 'fee' for magical or physical protection.
Races: Some locations could be the beginning of a race. You pay a fee and icons appear about the globe, which the player has to locate and activate in order, as quickly as possible. Maybe each location indicates the direction to the next one, or maybe some have monsters defending them? Perhaps some races would prevent usage of Potions while the race is in progress. Completion could give a reward in Gold or XP or items, based on a target time, or maybe a sliding scale depending on how fast the players is?

I guess I had some thoughts about the grind that seems to occur after Level 10 - things seem to slow down dramatically and it's hard to advance in XP without just hacking away at wanderers and hoping to get tough enough to take on the Level 4 Bosses. Not sure what could fix that as it's a bit boring at the moment. I'm not really sure I have the patience to grind away to get to Level 15 or higher if all there is to do is kill 5 more Bosses and get their crystals.
Player interaction could be achieved more if perhaps the crystals you capture need to be formed together into a final item? Maybe players would have to trade off to get the crystals they need (maybe it's random), or barter. I'm sure that would be quite difficult to achieve within the confines of Google Earth though.

That's all for now. Hope you enjoy some of the stuff in my crazed ramblings!