Epic boons are an interesting thing (that I've never yet used in 5e thus far). The biggest difficulty is that they are of varied power levels, with some being obviously far more powerful than others, while at the same time, some being far more useful for some classes than other classes (spellcasters seem to gain the advantage with epic boons far more than normal overall).
Of interest though, would you say someone could gain an epic boon more than once? For example, combat prowess, could they ever obtain it so they have he epic boon of combat prowess x2 (which still is nowhere close to being able to gain an additional 9th level slot that the spellcaster could gain...it may eventually be somewhat of a comparable boon I'd imagine)?
In regards to expanding the game otherwise, I did it rather easily when I created the 5e Old School document (shameless plug here) which is available on DMs Guild (still working on a Random Dungeon for solo player supplement though). I expanded XP tables, utilized tables from AD&D to extrapolate spell tables and other things. 5e makes it (and I'd say even encourages it) easy to do this if one so desires. No need to use my houserules (if I ever even got to that level of play, I've never played anything epic in 5e thus far), go and create your own. I think 5e encourages one to be the DM of their own game and govern/run it as they wish.
I don't know enough to know if they've statted Elminister for 5e, but I have no problems if he were a 27th (or 28th or 29th) level Magic-User. We may not know the rules for his stats, but the meaning is clear. We don't need to know his stats, he's an NPC and made that much more powerful simply so if we decide to do something stupid like attack him...he can destroy the entire party or put them someplace where an adventure awaits and they can't bother him...or any other such thing that Greenwood might desire.
The entire reason for them to be that high of level is NOT so we can see their stats (though it can be fun to do so) but to make it so that they are that all powerful type NPC that acts as the wise sage, or quest giver, or whatever other NPC role the DM needs, without having that same NPC open to be attacked and taken down.
If that's what you want though, there's nothing stopping one in 5e from going that route and creating an extension of the rules. If multiple epic boons of the same type are available, then it's even easier to stat out an Elminister (how many extra 9th level spells would he receive for example...and perhaps 40 more HP, etc...etc..etc).
I think 5e adheres more to keeping the same rules at times for NPC's than 4e did (and I actually like that, to be honest, some NPC's like Drizzt or others were rather ridiculous in my opinion in 4e, they should have been more like the PC's in regards to how they were statted out...and I liked 4e even) in my opinion, and because of that, doing things like what I said above in regards to characters like Elminister should actually be quite easy (once again, for the third time, in my opinion...obviously).