I don't see why it would be a tautology. As to whether that could be a workable basis for a normative ethical theory, that would depend entirely on how you fleshed out the rest of it. It sounds like it would be very similar to aretaic (virtue ethics) models, in that presumably you wouldn't just want more moral agents but more good moral agents, i.e. more laudable, virtuous people. It also sounds consequentialist in nature, in that you're focusing on the ends, without any regards to means so far. So a lot of critique of both utilitarianism (the premier consequentialist theory) and virtue ethics would probably apply to it.

In normative ethics, people resort to different systems like deontology, utilitarianism, rights, and virtues. — Peaceful Discord

System-less morality is the weakest part in western philosophy. It has the same propensity to infinite regress as metaphysics. I do not understand why people do not grasp the fundamentally inferior nature of such approach. It literally leads to nowhere at all.

If you ever studied even just the very basics of Jewish law or Islamic law, you would quickly see that western "ethics" is just a pile of nonsense. Morality requires the system-wide premises of a legitimate formal system.

I also assume you mean good moral agents. A moral agent is any rational being, and I doubt anyone would argue that increasing the number of people is a serious moral goal.

The wish to increase the number of virtuous people sounds like a viable combination of virtue ethics and utilitarianism. It wouldn’t be a rejection of either, but rather a confirmation of both.
In virtue ethics, virtue is the path to happiness and only a virtuous person can be truly happy. There is no contradiction between virtue ethics and utilitarianism and a believer in the former would naturally equate an increase of happiness with an increase of virtue.

However, such a moral goal cannot exhaust the goals of either ethical theory since not only virtue has an influence on happiness. Stealing and killing is wrong because thieves and murderers are less virtuous and therefore less happy, but obviously it is also wrong because someone who grieves the loss of life and property is less happy. No ethical theory can care about the agent only and ignore the moral patient since a moral wrong is always identified through someone suffering a moral wrong (at least potentially). The goal of any ethical theory would necessarily be to reduce both evil doing and evil suffering.

Is it possible for someone to form an ethical theory based on increasing moral agency? — Peaceful Discord

I think so. Here's a sketchy attempt ... and even sketchier clarification. (Embedded links go down rabbit holes of assumptions, etc.)

No ethical theory can care about the agent only and ignore the moral patient since a moral wrong is always identified through someone suffering a moral wrong (at least potentially). — Congau

True. Caring for a "moral patient", however, presupposes moral agency, whereby the latter increases in capabilities (as per OP's query) by exercising - a positive feedback loop - those capabilities which prevent, mitigate or relieve "suffering a moral wrong". Insofar as every moral agent also, simultaneously and always, is a moral patient (i.e. vulnerable or harmed), suffering is a "moral wrong" iff conceived of as decrease in - dysfunctioning of - capabilities which constitute agency (moral or otherwise); thus, an ethics of "increasing moral agency" entails reflectively caring for moral patients (i.e. cultivating virtue via negative utilitarian / consequentialist preferences and actions).

Thoughts?

entails reflectively caring for moral patients — 180 Proof

In principle, sure, and absolutely. But where the rubber meets the road, the moral patient and his need must be carefully identified - maybe not usually too much of a problem - and "caring" well-defined. The obvious example of problems created absent this rigour is caring for drug addicts. That is, it cannot even be done without careful preparatory work, understanding, and some preliminary decisions made and stuck to.

That is, if your program were all, then we can and should throw Kantian ethics out the window. If we must, we must, but I'm not yet feeling that must.

I don't follow. You're conflating 'personal morality' and 'public policy' - I think formulations & execution of the latter ought to be guided by principles derived from, and consistent with, the former but don't the former on that account as not an artifact, or consequence, of the latter. As for Kantian considerations, reformulated as

It is performatively self-contradictory for an agent to (actively) neglect, or not to take care, to develop - increase - her agency (moral and otherwise), therefore she must take care of her moral agency via exercises of caring for (i.e. preventing mitigating relieving the harm of) moral patients, — 180 Proof

the agent-based, aretaic-negative consequentialist hybrid I (sketchily) propose is, I think, grounded deontologically ... ('love child' of a Philippa Foot, Derek Parfit & Benny Spinoza ménage à trois :smirk:) ... so Kant's still relevant to me, however indirectly, for the time being.

I also assume you mean good moral agents. A moral agent is any rational being, and I doubt anyone would argue that increasing the number of people is a serious moral goal. — Congau

This is actually what a person I was debating claimed. And yes he did refer to an increase in the total number of moral agents instead of virtuous ones. I suppose we are all in agreement his system is absurd?

[That] It is performatively self-contradictory for an agent to.... is, I think, grounded deontologically ... so Kant's still relevant to me, however indirectly, for the time being. — 180 Proof

Me too. But the individual as agent never occurred to me. I had supposed that categorical imperatives were got out as necessary from the metaphysical/deontological lumber room, and if necessary, custom cut. That is, stored away until needed. But now it dawns on me - thank you I think - that in fact I am an ethical agent willy-nilly 24-7. My revenge? You are too!