I've been thinking about this a lot lately, too: LGB - T = ?

My conclusion (in-a-nutshell version): our emphasis should be on 1.] normalization/practicality; 2.] still-vulnerable groups (specifically, supporting them in achieving normalization, too).

And my conclusion in somewhat more fleshed-out form:

• Making LGB specifically about sexual orientation without making it hyper-sexualized. That is, no longer expecting it to comprise an entire culture in itself (on the order of Mexican-American culture or something), nor be about anything more than sexuality in its most basic form (that is, simply SSA orientation-- lesbian, gay, or bi-- rather than any and all expressions of sexuality). Seems to me that most of our problems trace back to this: trying to make just plain sexual orientation into more than it is. Because, unless artificially gussied-up? It's pretty elementary. A component. Of human sexuality and individual personality alike. Treating it as such makes for maximum inclusion of SSA people, since a wide range of actual cultures/worldviews (political affiliations among them) can then be accommodated. But if you want to go the sexuality = full-scale culture route, it naturally encourages the proliferation of any and all sexualities to fill up the space, resulting in hyper-sexualization and general creepiness. Let's get back to basics here: simplify and normalize. (This would also tend to keep us from attracting sex-freaks/creeps in the first place: we'd be too "boring" and lacking in "gay"-coded hairstyles, etc., to appropriate.)
• Minimalism: connected to the above-- try to keep LGB pared down to its essentials, so as to accommodate not only the widest range of SSA individuals, but all 3 letters, as fully as possible. Subgroups can represent specific constituencies (L, G, and B among them); at the "LGB" level, make it broad and inclusive. What do we all have in common? Focus on that.
• Practicality in general: what matters to LGB peoples' everyday lives; i.e., the boring real stuff. Normalcy is central here: establishing ourselves as just another part of society, in every respect, and only calling attention to our LGB-ness when it's relevant. Activists' goal should be making LGB advocacy a low-key, sensible, regularized thing: just keeping our hand in, as it were-- so the majority doesn't forget about us, and we have a say in decision-making. When you're a minority, it's important to make sure that your interests are represented and factored in whenever applicable.
• Identity: renewed commitment to what being LGB actually means. The definition needs to be clear (sexual orientation towards the same sex-- in the case of lesbians and gay men, exclusively-- without any reference to "gender") and non-negotiable. We need to insist on our right to our LGB/SSA identity, and be clear about how any attempt to override sex with gender is an attack on that identity, and therefore on us.
• Gatekeeping: related to the above. LGB is for SSA people only. Allies are welcome, but only as such. Otherwise straight people (being far more numerous) take over and we become marginalized in our own community.
• Focus on vulnerable groups: those LGB people still "left behind", despite our achievements overall. The goal should be to promote their strength and self-determination, not hype/perpetuate victimhood status. Who comes to mind: minors (esp. those abused and/or rejected by their families for being SSA); residents of the most anti-LGB countries (where oppression towards SSA people is widespread and often official); people traumatized by past bigotry in western nations (so that even if things around them are much improved, they still need help overcoming the psychological damage sustained earlier); those victimized by their fellow LGBs (sexual assault; domestic violence/abuse); those in poverty (often unable to defend themselves from bigotry). May, at least to some extent, also include women (bi and lesbian: subject to both sexism and homophobia); bi people (if recent studies indicating that they experience poorer mental & physical health/greater poverty than the LG are accurate); the elderly and/or disabled (often dependent on people unsympathetic to their sexuality); members of esp. homophobic religious and/or ethnic groups. Emerging addition to the above: LGB "de-trans" people.
• The B: since bisexuality remains to a great extent invisible, misunderstood, or dismissed altogether-- even among SSA people themselves-- it could probably use a bit more attention in the LGB. Not so much that it becomes the lgB, of course; just enough to give bisexuals parity. To this end: emphasis on seeing it as simply another sexual orientation (not being sexually-indiscriminate, confused, in denial, "open-minded"/"enlightened", or anything else), and not through a monosexual lens; spreading awareness of these things, and of its very existence.
• Gay men and hate crimes: since gay men remain the primary targets for homophobic violence, this deserves special attention. Along with the more obvious measures, working to reduce misogyny and the influence of gender-roles in society is a must (so men perceived as being "like women" no longer provoke outrage).

Well, though exhausting, that's still hardly exhaustive, is it? :) Anyway, these are the things of LGB importance that occur to me.

TL;DR: emphasis on LGB serving rank-and-file SSA people by facilitating our normalization, maximizing inclusiveness, protecting our identity, and focusing on practical real-life issues/concerns (esp. those of the most vulnerable LGB people).