I like the idea of having level advancement and increasing attributes separate. Neither 5th nor 7/7.5 editions of T&T really satisfied me in this regard so I developed my own system combining the methods for character levels and improving attributes established in these edition. Of course this system does have some drawbacks in the bookkeeping requirements.
Recently a new discussion at the Trollbridge related to this topic was started by Kwll where he proposed basing character level not on the value of any one attribute as in 7/7.5 edition, but simply on the number of times the character has improved any attributes.
In thinking about this system I realized that there is a simpler approach to my own system which still combines elements of 5th and 7.5 edition and requires much less bookkeeping.
Individual attributes are increased using the 10x current attribute system of the 7.5 edition. Keep track of how many APs are spent to increase your attributes (don't worry about how those APs were earned as in my previous system). Once this number reaches the number of APs specified in 5th edition for each level (1000 AP = 2nd level; 3000 AP = 3rd level; 7000 AP = 4th level; etc) you gain a level. The caveat would be that these APs would have to be spent to count, not just be sitting in the "bank." So you just need to keep track of two numbers, the number of AP gained and number of AP spent.
Increasing attributes has obvious advantages of increasing your character's general abilities, strength, dexterity, intelligence, luck, etc., which are not necessarily related to the character's profession but will improve your character's chances of defeating opponents, making it past traps, and basically surviving. I do prefer this method of increasing attributes over that of 5th edition where only one or two attributes are increased with each new level. It gives players much more flexibility in crafting their characters over time. It certainly gives them a lot more options than raising their Luck by 4 once they reach second level. It also makes attributes and levels distinct, each offering something different because they are different.
When your character gains a level, he/she gains the new level bonuses according to type: level # bonus warrior adds, decreased spell cost for wizards, and level # bonus to Roguery talent for rogues (Note: this is a house rule of my own since rogues have no specific level bonus in 7.5) as well as general bonuses for all types: level # saving roll bonus and one new talent.
This system still separates attribute values and character levels since the value of your attributes does not matter as long as you have earned and spent the requisite number of adventure points. This is important because if you start off with a lot of bad attribute rolls when you create a character, you don't have to wait until you finally raise one of them to 20 to increase your abilities as a warrior, rogue, or wizard. You simply need to earn and spend 1000 adventure points, just like that other character that started with a Luck of 18. It also has the advantage of eliminating the chance of characters starting at 2nd level or higher at their creation, like every Dwarf on the planet.
I plan on play testing this new system in my next T&T play-by-post game to see how it works and if players find it worthwhile. As always, I welcome any comments or suggestions.