I had similar ideas and was thinking of placing a separate request for this, but I'm glad I stumbled upon this here. The important thing to note with this type of change is that it needs to be exclusive to Endless mode, since the role the Balance modifier currently has in Normal mode is healthy. I haven't quite done the math yet, but I do know that the Normal tower's Ultimate ability may distribute up to 1000xp/sec to its clones based on observation. Now while this is obviously excellent for both power and XP distribution purposes, the main reason these changes are needed are because pulling such a feat off requires a significant amount of space economy and a miner to help support XP intake, things not every location has. The Normal tower also is unable to distribute its XP to multiple different targets, which makes XP transfer more difficult in some scenarios. Additionally, I have found that I can barely get a tower to level 100 in over 100 waves of constant level five Balance modifier uptime even while linked to a level 150 tower being fed by an XP modifier.

So, in order to assist with allowing the Balance modifier to reach the heights the Normal Ultimate ability can reach without depriving it of its role, I would additionally propose two new Endless-specific research options, which can be implemented together or separately. These upgrades should be expensive (250 bit dust minimum) but not egregiously so (e.g. Powerful tower ability expensive, which is 1k+ bit dust each) to make them obtainable but not so difficult to grind for them that they effectively necessitate a research ticket.

1. Increase maximum upgrade level by 1, maximum 5. This allows the Balance modifier to reach level 10, at which point the tower would be able to transfer over 1000xp/sec. However, as a drawback and to provide the Basic tower a clear advantage in the matter, its base XP cost would be somewhere on the order of 100,000xp to ensure that players who want to upgrade the modifier to this extent really want to do so. The only times such an advantage would manifest would be during a very deep Endless run, which is why this would synergize well with...

2. Increase granted level cap by 0.2, maximum 5 to a multiplier of 2.0. The Balance modifier will now grant additional levels on top of having more levels it can upgrade into, granting level 6 at Balance level five, granted cap level 1, and up to 20 additional levels at Balance level 10, max upgrade level 5, granted level cap level 5. This is an incredibly powerful upgrade that should be even more expensive to upgrade than the maximum level increase upgrades due to lack of downsides. This allows for the further utilization of Balance modifiers as a less efficient version of the Normal tower due to granting greater levels of PWR to towers while not necessarily eclipsing a max level Normal tower. It also allows for players to chase the fantasy of having a ridiculously overleveled tower beyond what can be currently granted outside of level 6.4, which can create all sort of ridiculous but fun(nel) strategies that will help add to the gameplay loop.

In reference to the previous note, these changes would likely be so significant as to require them to be, in part or in whole, relegated to season 4 due to how they would inherently warp certain strategies. Their Endless-specific nature may still allow for them to be implemented, but it is something to keep in mind.