It is, of course, possible that Loch Lomond is not the correct lake and the fame of its many islands is derived from a mistaken identification with the description of the wonder.
One interesting Welsh alternative would seem to be the lake mentioned by Gerald of Wales* (1197 CE: Book 1, Chapter II [throughout]):
It came to pass before that great war, in which nearly all this province was destroyed by the sons of Jestin, that the large lake, and the river Leveni, which flows from it into the Wye, opposite Glasbyry, were tinged with a deep green colour. The old people of the country were consulted, and answered, that a short time before the great desolation caused by Howel, son of Meredyth, the water had been coloured in a similar manner...It is an ancient saying in Wales, that if the natural prince of the country, coming to this lake, shall order the birds to sing, they will immediately obey him...The lake also (according to the testimony of the inhabitants) is celebrated for its miracles; for, as we have before observed, it sometimes assumed a greenish hue, so in our days it has appeared to be tinged with red, not universally, but as if blood flowed partially through certain veins and small channels. Moreover it is sometimes seen by the inhabitants covered and adorned with buildings, pastures, gardens, and orchards. In the winter, when it is frozen over, and the surface of the water is converted into a shell of ice, it emits a horrible sound resembling the moans of many animals collected together; but this, perhaps, may be occasioned by the sudden bursting of the shell, and the gradual ebullition of the air through imperceptible channels.
Hoare* notes that this is likely to be Llangors(e) Lake in the Brecon Beacons (alternative names: Llyn Savaddan [Welsh]; Brecenanmere [Anglo-Saxon] /Brecinau-mere; and Talyllyn Pool) (Visit Details). The river is now known as the Llynfi (pronounced Lh-un-vi?). He notes that the locals believe it to have swallowed up an ancient city. Camden gives further details and speculations:
Leveney a little river, after it is run into this Poole, keepeth his owne hew and colour still by himselfe, as disdayning to be mingled therewith (which the very colour sheweth), is thought to carry out his owne water, entertained a while there by the way, and no more than he brought in with him. It hath beene a currant speech of long continuance among the neighbours there about, that where now the Meere is there was in times past a City, which being swallowed up in an earth quake, resigned up the place unto the waters. And beside other reasons, they allege this for one, that all the high waies of this shire come directly hither on every side. Which if it be true, what other City should a man thinke stood by the river Leveney than Loventium, which Ptolomee placeth in this tract? And in no place hitherto could I finde it (albeit I searched diligently for it) either by the name, or situation, or ruines remaining. Marianus Scotus (which I had almost forgotten) seemeth to call this Lake Bricenau Meere, who recordeth that Edelfled the Mercian Lady in the yeere 913 entred into the land of the Britans to win by assault a Castle at Bricenau-Meere, and that she tooke there the King of the Britans wife prisoner. Whether this Castle were Brechnock it selfe or Castle Dinas, which standeth over it upon a rockey hill, and which the higher it riseth the slenderer and smaller it becommeth, is not certainly knowen. But that Blean Leveney Castle hard by was the chiefe place of the Barony that Petre Fitz-Herbert the sonne of Herbert Lord of Dean-forest by Lucy the daughter of Miles Earle of Hereford held, appeareth evidently upon Record. [Camden, with interpolations by Holland*: Brechnockshire]
Llangors is the largest (or second largest) natural lake in South Wales and is naturally eutrophic* (i.e. over-rich in nutrients hence prone to algal blooms, which can be red or green). In the locals' defence it did have a crannog dwelling in the centre of it (see Myth page on crannogs, and Benfield and McKewan*), built 889-893 CE and probably burnt down by the Anglo-Saxons in 916 CE*. The combination of wise birds, a fortified island, and a marvellous nature is almost too much to be ignored (see Myth page) given the large advantage of being near the other wonders early in the list, which are all in the Welsh borders around the Wye and Severn. However, the strong name-link of our wonder with Lomond would need addressing. Might we, very speculatively, suggest that the initial position was occupied by Llangors, but the similarity of river names caused someone to suggest its replacement? In the absence of any evidence it seems pointless to do anything but hold it in mind. No evidence, for example, as come to light suggesting the lake was called Lumon. Although the Brecon Beacon's have a fairly long history of having the "Beacon" name (at least to the 17th C.?*), and this might suggest an alternative name for the lake is possible (See Note on Meaning of Lumon), there is no evidence for the lake having been named in this manner, indeed, the Book of Llan Dâv (See Other Works) has the boundary of Llan Cors as including lake Syuadon and the river Lyfni ("Levni"?), suggesting that while the river name was closer to that in the Wonders, the lake name was very different.
There are two less likely lochs in Scotland - the Loch Leven in Fife that drains through the River Leven to the North Sea, and the tidal Loch Leven near Fort William, which has a River Leven which drains into, not out of, it. Morris*, for example, has "Loch Leven" as a translation. Note, however, that this has also been a name for Loch Lomond in the past (See Note on Lomond).