By making three relatively simple additions to the game, which require little in the way of programming, the game can be substantially improved.
1. Implement a leveling system via heat. This could be done in several ways:
a. While gaining heat you expand the number of stars your equipment can have. You start at 5 stars (meaning you can wear 1 star armor, helmet, shield, and two weapons) and level your way up from there.
b. Add a potential limiter based on heat level, similar to the effect of shallow depths on high star equipment. Even at depth 28, 5 star gear won't be effective unless your heat level is high enough.
Obviously characters with tier 2 or 3 activated you'd need to give a higher starting heat level. Perhaps give them enough heat level to be able to equip all their currently equipped gear. For the star method, and possibly the limiter method, make it slow down quite a bit at higher level so people have to work to get their 3rd and 4th 5* weapon as well as 5* trinkets.
2. To go along with suggestion one, add an Incinerator either in town or at random in the Clockworks where a person can burn a certain amount of materials a day to transform them into heat. Give more heat for higher star mats. Give even more heat for burning equipment.
This should alleviate some of the material glut and make materials more important to people. The extra equipment from crafting for UV's would also have a use as well.
3. At tier 2, allow people to specialize in a weapon type. This could give reduced charge rate, damage, or weapon speed (or something special, like drastically speeding up reloading guns or allow you to shield during reloading). At tier 3, allow people to have a secondary, weaker specialization in another weapon type. Allow respecs, but make them cost fairly high energy levels, or have the amount increased each time to discourage it.
This will give meaning to having multiple characters. I know this goes against the classless idea of Spiral Knights, but I believe it would drastically lengthen and improve the game. Some people enjoy feeling specialized at certain things, though it might be necessary to offer a 'generalist' specialization as well. Having multiple characters will mean needing to equip them as well as raise their heat levels.
These are all easy programming tasks that require almost no graphics updates (other than building an incinerator and a few menus, perhaps). They improve game length drastically, give purpose to heat, reduce mat glut, and create a greater demand for equipment.
What do you think?
None of these sound fun.
I very much enjoy being able to invite new people to the game and give them powerful equipment to join me on dungeon runs. Having to say "well, uh, just do a bunch of floors until you can keep up with me. And here, have a bunch of my old materials to burn for heat." does not sound very fun at all.
The lack of experience points and character level in this game is a deliberate design choice, and it's one that I and my guild have found refreshing and enjoyable.
I mean, you say you want to extend the game... But the game is essentially the same at T1 as it is at T3. If you don't enjoy the basic game-play of fighting through dungeons, hitting switches, and weaving through elegant danmaku-firing turret arrays, you won't enjoy it, even if it takes ten times as long to make the same numbers pop up when you hit something.